Simultaneous turnover studies of thyroxine, 3,5,3' and 3,3',5'-triiodothyronine, 3,5-, 3,3'-, and 3',5'- diiodothyronine, and 3'-monoiodothyronine in chronic renal failure

The present study evaluates the sequential extra-thyroidal monodeiodination of thyroid hormones through tri-, di-, and monoiodothyronines in chronic renal failure (CRF) in man. Simultaneous turnover studies of T4, T3, rT3, 3,5-diiodothyronine (3,5-T2), 3,3'-T2, 3',5'-T2, 3'5'-T2, and 3'-monoiodothyronine (3--T1) were conducted in six patients with CRF (creatinine clearance, 9-18 ml/min) using the single-injection, noncompartmental approach. Serum levels of T4, T3, and 3,5-T2 were reduced to two thirds of control levels (P less than 0.05), whereas serum rT3 and 3,3'-T2 levels were reduced to a minor degree. Serum 3'-5'-T1 was doubled (p less than 0.05). The MCRs of T4, rT3, and 3',5'-T2 were enhanced to 168%, 127%, and 187% of normal (P less than 0.05), respectively, whereas those of T3, 3,5-T2, 3,3'-T2, and 3'-T1 were unaffected. The mean production rates (PRs) of the iodothyronines in CRF were as follows (CRF vs. control values, expressed as nanomoles per day/70 kg): T4, 119 vs. 125; T3, 26 vs. 44 (P less than 0.01); rT3, 49 vs, 48; 3,5-T2, 3.5 vs. 7.2 (P less than 0.001); 3,3'-T2, 25 vs. 35 (P less than 0.01); 3',5'-T2, 25 vs. 14 (P less than 0.01); and 3'-T1, 39 vs. 30. Previous studies have demonstrated reduced phenolic ring (5'-) deiodination of T4 in CRF, which is supported by the present finding of unaltered PR of T4 and reduced PR of T3. In contrast the 5'-deiodination of T3 leading to the formation of 3,5-T2 was found unaffected by CRF, since the conversion rate (CR) of T3 to 3,5-T2 (PR 3,5-T2/PR T3) was unaltered (16% vs. 15% in controls). The tyrosylic ring (5-) deiodination of T4 to rT3 was unaffected in patients with CRF, the CR being 42% vs. 40% in controls, in contrast to an enhanced CR of rT3 to 3',5'-T2 (53% vs. 29%, P less than 0.01), which also is a 5-deiodination step. In conclusion, our data show that CRF profoundly changes the kinetics of all iodothyronines studied. Furthermore, our data are compatible with the existence of more than one 5'-deiodinase as well as more than one 5-deiodinase in man.     

Splanchnic extraction of 3,3'-diiodothyronine and 3',5'-diiodothyronine in hyperthyroidism

The splanchnic extraction of 3,3'-diiodothyronine (3,3'-T2) and 3',5'-diiodothyronine (3',5'-T2) was studied in 7 hyperthyroid patients and 20 normal subjects employing the hepatic venous catheterization technique. A significant net uptake by splanchnic tissues was found for both diiodothyronines . The fractional splanchnic extraction calculated as the arterio-hepatic venous plasma concentration difference divided by the arterial concentration was unaffected by hyperthyroidism as compared to normal values. There was a close positive correlation between the arterio-hepatic venous concentration difference and arterial concentration, 3,3'-T2: r = 0.988, and 3',5'-T2: r = 0.932 (P less than 0.001). The splanchnic extraction was nonsaturable at endogenous plasma concentrations of 3,3'-T2 up to at least 17.0 ng/dl and of 3',5'-T2 up to at least 15.2 ng/dl. The data suggest that the splanchnic extraction of 3,3'-T2 and 3',5'-T2 obeys first order kinetics, the fractional extraction being unaffected by hyperthyroidism. Furthermore, changes in the net splanchnic extraction of 3,3'-T2 and 3',5'-T2 do not seem to contribute to changes in circulating levels of these iodothyronines. It is suggested that tissues other than the liver contribute significantly to the deiodination process both in normal and in hyperthyroid man.     

Secretion of thyroxine, 3,5,3'-triiodothyronine and 3,3'5'-triiodothyronine in euthyroid man

The secretion of iodothyronines from the normal human thyroid gland was assessed by radioimmunoassay analyses of the concentrations of thyroxine (T4), 3,5,3'-triiodothyronine (T3) and 3,3',5'-triiodothyronine (reverse T3, rT3) in thyroid venous and peripheral venous blood. The subjects studied were euthyroid patients undergoing parathyroid surgery. Measurements were carried out both under apparently normal conditions, following peroral T3 pre-treatment, and before and after acute administration of TSH into a thyroid artery. In the control subjects, significant gradients between thyroid venous and peripheral venous concentrations were recorded both for T4, T3 and rT3, suggesting that all three iodothyronines are secreted by the normal human thyroid. T3 pre-treatment seemed to reduce this secretion, and acute administration of TSH promoted rapid, marked, and concomitant increments in the thyroid venous concentrations of all three iodothyronines. Hence, it appears that not only T4 but also T3 and rT3 are secreted by the normal human thyroid gland, and that TSH stimulates the secretion of all three iodothyronines. On the other hand, calculations of the relative secretion rates uielded the relation T4:T3:rT3 as 85:9:1. This indicates that, in euthyroid subjects, most of T3, and almost all of rT3, is produced by extrathyroidal conversion of T4 and not by direct thyroidal secretion.     

Phenolic and tyrosyl ring iodothyronine deiodination by the Caco-2 human colon carcinoma cell line

Thyroid hormone metabolism was studied in the human Caco-2 colon carcinoma cell line, which at confluence exhibits several functions of differentiated enterocytes. Cells were harvested two to 17 days after reaching confluence. Intact cells and homogenates were tested for deiodination of [125I]-labeled substrates. Small amounts of thyroxine (T4) were converted by homogenates to 3,3',5'-triiodothyronine (rT3), 3,3'-diiodothyronine (3,3'-T2), and 1-, with no detectable production of 3,5,3'-triiodothyronine (T3) by homogenates or cells. rT3 was converted to 3,3'-T2 and 1- with an apparent Michaelis constant (Km) for rT3 of 24 nmol/L; 6-n-propyl-2-thiouracil (PTU) had a 50% inhibitory concentration of 30 nmol/L and abolished rT3 5'-deiodination at 1 mmol/L in the presence of 20 mmol/L dithiothreitol (DTT). T3 was deiodinated to 3,3'-T2 and 3'-monoiodothyronine (3'-T1) with an apparent Michaelis constant (Km) for T3 of 5.7 nmol/L; this reaction was not inhibited by 1 mmol/L PTU. Phenolic and tyrosyl ring deiodinating activities were maximal four and six days, respectively, after the cells reached confluence. Homogenates of cells grown in standard medium containing fetal calf serum had fivefold higher rT3 5'-deiodinating activity than cells grown in a serum-free defined culture medium, reflecting a fivefold difference in the apparent Vmax with no difference in the apparent Km for rT3. There was no difference in T3 5-deiodination rates in homogenates of Caco-2 cells grown in the two media until 12 days postconfluence, when cells grown in standard medium had higher activity.(ABSTRACT TRUNCATED AT 250 WORDS)     

Plasma thyroxine, 3,3',5-triiodothyronine and 3,3',5'-triiodothyronine during beta-adrenergic blockade in hyperthyroidism

Plasma thyroxine (T4), 3,3',5-triiodothyronine (T3) and 3,3',5'-triiodothyronine (rT3) were measured in 16 patients with Graves' disease. Patients were studied under the following conditions: first without any treatment, then, during beta-adrenergic blockade with propranolol, and finally after euthyroidism had been attained by carbimazole. During propranolol T3/T4 ratio decreased, whereas T4 remained unchanged. After carbimazole T3/T4 ratio returned to its pretreatment value. rT3/T4 ratio showed opposite changes. These results suggest that peripheral conversion of T4 into T3 and rT3 in hyperthyroidism is, at least partly, dependent on the functional status of the beta-adrenergic system. Suppressed peripheral conversion of T4 into T3 during beta-adrenergic blocking agents may contribute to the beneficial effects of these drugs in thyrotoxicosis.     

Metabolism of 3,3'-diiodothyronine in rat hepatocytes: interaction of sulfation with deiodination

Production of 3,3'-diiodothyronine (3,3'-T2) is an important step in the peripheral metabolism of thyroid hormone in man. The rapid clearance of 3,3'-T2 is accomplished to a large extent in the liver. We have studied in detail the mechanisms of this process using monolayers of freshly isolated rat hepatocytes. After incubation with 3,[3'-125I]T2, chromatographic analysis of the medium revealed two major metabolic routes: outer ring deiodination and sulfation. We recently demonstrated that sulfate conjugation precedes and in effect accelerates deiodination of 3,3'-T2. In media containing different serum concentrations the cellular clearance rate was determined by the nonprotein-bound fraction of 3,3'-T2. At substrate concentrations below 10(-8) M 125I- was the main product observed. At higher concentrations deiodination became saturated, and 3,3'-T2 sulfate (T2S) accumulated in the medium. Saturation of 3,3'-T2 clearance was found to occur only at very high (greater than 10(-6)M) substrate concentrations. The sulfating capacity of the cells exceeded that of deiodination by at least 20-fold. Deiodination was completely inhibited by 10(-4) M propylthiouracil or thiouracil, resulting in the accumulation of T2S while clearance of 3,3'-T2 was little affected. No effect was seen with methimazole. Hepatocytes from 72-h fasted rats showed a significant reduction of deiodination but unimpaired sulfation. Other iodothyronines interfered with 3,3'-T2 metabolism. Deiodination was strongly inhibited by 2 microM T4 and rT3 (80%) but little by T3 (15%), whereas the clearance of 3,3'-T2 was reduced by 27% (T4 and rT3) and 12% (T3). It is concluded that the rapid hepatic clearance of 3,3'-T2 is determined by the sulfate-transferring capacity of the liver cells. Subsequent outer ring deiodination of the intermediate T2S is inhibited by propylthiouracil and by fasting, essentially without an effect on overall 3,3'-T2 clearance.     

The effects of phenytoin (diphenylhydantoin) on the extrathyroidal turnover of thyroxine, 3,5,3'-triiodothyronine, 3,3',5'-triiodothyronine, and 3',5'-diiodothyronine in man

The extrathyroidal metabolism of T4, T3, rT3, and 3',5'-diiodothyronine (3',5'-T2) was studied before and after treatment with 350 mg phenytoin (DPH) daily for 14 days in six hypothyroid patients receiving constant L-T4 replacement. The total and free serum concentrations of the four iodothyronines were reduced by approximately 30% during DPH treatment, whereas the free fractions in serum were unaltered. Concomitantly, serum TSH increased 137% (P less than 0.02). The production rate (PR) of T4 decreased 16% (P less than 0.005), indicating decreased intestinal absorption (bioavailability) of oral L-T4 during DPH treatment. The fractional rate of 5'-deiodination of T4 to T3 increased from 27% to 31% (P less than 0.05), whereas the rate of 5-deiodination of T4 to rT3 decreased from 45% to 25% (P less than 0.05). The urinary excretion of free and conjugated T4 was 2.3% of the T4 PR and was unaffected by DPH. Thus, the amount of T4 metabolized through nondeiodinative pathways apart from urinary excretion increased from 25% to 44% (P less than 0.05). The apparent distribution volume (Vd) of T4 increased (P less than 0.05), whereas the pool size was unchanged. The PR of T3 did not change during DPH treatment, nor did the mean transit time or the cellular clearance. The rT3 PR was reduced by 54% (P less than 0.02) during DPH treatment. Concomitantly, the transit time increased 10-fold (P less than 0.05), whereas Vd and pool size increased 5-fold (P less than 0.01 and P less than 0.05, respectively). The turnover of 3',5'-T2, in contrast to that of the other iodothyronines, did not change significantly during DPH treatment. T3 formation from T4 was measured in liver microsomal fractions from rats treated for 8 days with DPH and was almost identical to that in untreated animals. The data demonstrate that DPH in therapeutic concentrations did not affect serum protein binding of the iodothyronines. DPH reduced the intestinal absorption of T4 and increased the nondeiodinative metabolism of T4. The resulting decrease in total and free serum T4 and T3 was associated with an increase in serum TSH, demonstrating reduced negative feedback on the pituitary. Our data do not support the assumption that DPH induces increased hepatic deiodinating enzyme activity.(ABSTRACT TRUNCATED AT 400 WORDS)     

AMNIOTIC FLUID CONCENTRATIONS OF 3,3'',5''-TRI-IODOTHYRONINE (REVERSE T3), 3,3''-DI-IODOTHYRONINE, 3,5,3''-TRI-IODOTHYRONINE (T3) AND THYROXINE (T4) IN NORMAL AND COMPLICATED PREGNANCY

Amniotic fluid concentrations of 3,3',5'-tri-iodothyronine (rT3), 3,3'-Di-iodothyronine (3,3'-T2), 3,5,3'-tri-iodothyronine (T3) and T4 were studied in 384 women during normal and complicated pregnancy. An inverse correlation was observed between decreasing rT3 and increasing 3,3'-T2 concentrations in amniotic fluid with gestational age. The mean rT3 level in normal pregnancy was 2.81 nmol/1 at 12-20 weeks and decreased significantly to 1.06 nmol/1 at 36-42 weeks of gestation. The mean 3,3'-T2 concentration was 49.1 pmol/1 at12-20 weeks increasing to 119 pmol/1 at 36-42 weeks. The mean T4 value of 3.83 nmol/1 at 12-20 weeks was about half that of later periods. The T3 concentration in a random sample of 45 amniotic fluids ranged from less than 28 to 370 pmol/1 (mean 102 pmol/1). The mean rT3, 3,3'-T2 and T4 values measured in patients with intra-uterine malnutrition, gestation diabetes, tocolysis, placental insufficiency and rhesus incompatibility at 31-40 weeks of gestation were not significantly different from those in uncomplicated pregnancy. Significantly decreased rT3 and T4 concentrations were found in toxaemia. From the results obtained in complicated pregnancy it may be concluded that measurements of iodothyronines, especially rT3, in amniotic fluid have insignificant diagnostic value in the recognition of intra-uterine lesions with the probable exception of fetal hypothyroidism. The analysis of the dependence of iodothyronine concentrations on the gestational age showed a maximum of rT3 and T4 levels between 20 and 30 weeks of pregnancy. This marked rise of iodothyronine concentrations in amniotic fluid at mid-gestation may be due to the onsetting maturation of the hypothalamic-pituitary-thyroid control system of the fetus.     

Altered serum levels of thyroxine, triiodothyronines and diiodothyronines in endogenous depression

Serum levels of thyroxine (T4), 3,3',5-triiodothyronine (T3), 3,3',5-triiodothyronine (rT3), 3,5-diiodothyronine (3,5-T2), 3,3'-diiodothyronine (3,3'-T2) and 3',5'-diiodothyronine (3',5'-T2) were studied in 80 patients with endogenous depression before and after electroconvulsive treatment (ECT). Compared to the values found after recovery, the patients when depressed had significant increased serum levels of T4, rT3, 3,3'-T2 and 3',5'-T2. Serum concentrations of T3 and 3,5-T2 were not significantly altered. Similarly the free T4 index (FT4I) was increased, while the free T3 index (FT3I) was unaffected. Previous studies have shown a reduced TSH response to TRH in patients with endogenous depression and that the long-term outcome after ECT is strongly related to changes in the TSH response. However, patients with increased TSH response to TRH (n = 23) had a pattern of serum iodothyronine concentrations similar to those (n = 57) with an unchanged TSH response. A similar pattern was also found in 7 patients with nonendogenous psychosis, in whom the TSH response to TRH was unchanged after recovery. It is concluded that the alterations of the TSH response to TRH found in endogenous depression cannot be explained by changes of FT4I or FT3I.     

Characterization of human liver thermostable phenol sulfotransferase (SULT1A1) allozymes with 3,3',5-triiodothyronine as the substrate.

Sulfotransferase 1A1 (SULT1A1) (thermostable phenol sulfotransferase, TS PST1, P-PST) is important in the metabolism of thyroid hormones. SULT1A1 isolated from human platelets displays wide individual variations not only in the levels of activity, but also in thermal stability. The activity of the allelic variant or allozyme SULT1A1*1, which possesses an arginine at amino acid position 213 (Arg213) has been shown to be more thermostable than the activity of the SULT1A1*2 allozyme which possesses a histidine at this position (His213) when using p-nitrophenol as the substrate. We isolated a SULT1A1*1 cDNA from a human liver cDNA library and expressed both SULT1A1*1 and SULT1A1*2 in eukaryotic cells. The allozymes were assayed using iodothyronines as the substrates and their biochemical properties were compared. SULT1A1*1 activity was more thermostable and more sensitive to NaCl than was SULT1A1*2 activity when assayed with 3,5,3'-triiodothyronine (T(3)). Sensitivities to 2,6-dichloro-4-nitrophenol (DCNP) and apparent K(m) values for SULT1A1*1 and for SULT1A1*2 with iodothyronines were similar. Based on K(m) values, the preferences of these SULT1A1 allozymes for iodothyronine substrates were the same (3,3'-diiodothyronine (3,3'-T(2))>3', 5',3-triiodothyronine (rT(3))>T(3)>thyroxine (T(4))>3,5-diiodothyronine (3,5-T(2))). SULT1A1*1 activity was significantly higher than the SULT1A1*2 activity with T(3) as the substrate. Potential differences in thyroid hormone sulfation between individuals with predominant SULT1A1*1 versus SULT1A1*2 allozymes are most likely due to differences in catalytic activity rather than substrate specificity.     

Synthesis of thyroid hormone metabolites by photolysis of thyroxine and thyroxine analogs in the near UV.

Photolysis of thyroxine and its analogs in the near UV permitted synthesis in good yield of picogram to gram quantities of thyroid hormone metabolites. Preparation of the same metabolites by classical chemical synthesis requires multistep procedures. Specifically labeled metabolites of high specific activity (e.g., those carrying the label in the nonphenolic ring) were obtained by photolysis of appropriately labeled thyroxine or 3',3',5'-triiodothyronine (reverse triiodothyronine). Some of these labeled metabolites, which are required for metabolic studies (3-iodothyronine and 3,3'-diiodothyronine, labeled in the nonphenolic ring), had not previously been obtained by other methods. Irradiation of thyroxine and reverse triiodothyronine in 150 mM methanolic ammonium hydroxide with greater than 340-nm light caused removal of one iodine atom from the phenolic ring with formation of 3,5,3'-triiodothyronine and 3,3'-diiodothyronine, respectively. Irradiation with higher-energy light (greater than 300 nm) led to stepwise removal of additional iodine atoms. Those in the phenolic ring were removed preferentially, so that 3,5-diiodothyronine and 3-iodothyronine, respectively, were formed. The iodine atoms in the nonphenolic ring were lost more slowly. Tetraiodothyroacetic acid followed a similar photodeiodination pattern. Photolysis with light in the near UV is a simple method for the synthesis of thyroid hormone metabolites.     

Comparative aspects of the distribution, metabolism, and excretion of six iodothyronines in the rat

We have studied the kinetics of 3 iodothyronines, 3,3'-diiodothyronine (T2), 3',5'-T2, and 3'-monoiodothyronine (T1), in groups of young adult male rats maintained under normal steady state physiological conditions. We have also performed a comparative analysis of these results, combined with corresponding kinetic indices of T4, T3, and rT3, to obtain a more comprehensive understanding of normal thyroid hormone production, distribution, and metabolism. Tracer doses of 125I-labeled 3,3'-T2, 3',5'-T2, and 3'-T1 were separately injected iv, and blood samples were collected 6-12 times for each iodothyronine in optimized sequential kinetic studies designed to maximize the precision of kinetic parameters. Labeled iodothyronines were separated quantitatively from their metabolites in each plasma sample by Sephadex G-25 column chromatography. Conventional kinetic analysis of the resulting data generated distribution volume, clearance, turnover, and mean residence time indices for each iodothyronine, and concomitant compartmental analysis of the same data provided additional results useful for integration and comparative analysis of the 6 iodothyronines. Kinetic parameters for all but T4 and T3 were similar, suggesting that similar mechanisms are responsible for the transport, metabolism, and distribution of nonhormonal iodothyronines. All but T4 and T3 (and, to a much lesser extent, 3'-T1) were almost completely and irreversibly metabolized, whereas 24-30% of the hormones (and 6% of 3'-T1) were excreted as such in feces only. Three-pool models fitted individual plasma kinetic data sets best in all cases (for all 6 iodothyronines), each with a plasma, a slowly exchanging (slow), and a rapidly exchanging (fast) pool, and kinetic parameters of interest were quantified for each iodothyronine (Ti). Quantitative analysis of an integrated 18-pool model for all 6 Tis revealed several other features of physiological interest. The fractional transport rate of T3 into the fast pool (liver, at least) is about an order of magnitude larger than that for all other Tis, supporting the hypothesis that transport of T3 into fast tissues (e.g. liver cells) is selectively amplified relative to that of the 5 other iodothyronines studied. Simultaneous and direct comparison of the 6 plasma kinetic data sets also supports this result. In addition, composite slow tissue pools, which probably exclude liver and kidney, contained the largest whole body fractions of all Tis (greater than 50%), and these also appear to be major sites of whole body T4 monodeiodinations.(ABSTRACT TRUNCATED AT 400 WORDS)     

Thyronamines are isozyme-specific substrates of deiodinases

3-Iodothyronamine (3-T 1 AM) and thyronamine (T AM) are novel endogenous signaling molecules that exhibit great structural similarity to thyroid hormones but apparently antagonize classical thyroid hormone (T(3)) actions. Their proposed biosynthesis from thyroid hormones would require decarboxylation and more or less extensive deiodination. Deiodinases (Dio1, Dio2, and Dio3) catalyze the removal of iodine from their substrates. Because a role of deiodinases in thyronamine biosynthesis requires their ability to accept thyronamines as substrates, we investigated whether thyronamines are converted by deiodinases. Thyronamines were incubated with isozyme-specific deiodinase preparations. Deiodination products were analyzed using a newly established method applying liquid chromatography and tandem mass spectrometry (LC-MS/MS). Phenolic ring deiodinations of 3,3',5'-triiodothyronamine (rT3AM), 3',5'-diiodothyronamine (3',5'-T2AM), and 3,3'-diiodothyronamine (3,3'-T2AM) as well as tyrosyl ring deiodinations of 3,5,3'-triiodothyronamine (T3AM) and 3,5-diiodothyronamine (3,5-T2AM) were observed with Dio1. These reactions were completely inhibited by the Dio1-specific inhibitor 6n-propyl-2-thiouracil (PTU). Dio2 containing preparations also deiodinated rT(3)AM and 3',5'-T2AM at the phenolic rings but in a PTU-insensitive fashion. All thyronamines with tyrosyl ring iodine atoms were 5(3)-deiodinated by Dio3-containing preparations. In functional competition assays, the newly identified thyronamine substrates inhibited an established iodothyronine deiodination reaction. By contrast, thyronamines that had been excluded as deiodinase substrates in LC-MS/MS experiments failed to show any effect in the competition assays, thus verifying the former results. These data support a role for deiodinases in thyronamine biosynthesis and contribute to confining the biosynthetic pathways for 3-T 1 AM and T 0 AM.     

The effects of Soybean Diet on Thyroid Hormone and Thyrotropin Levels in Aging Rats

OBJECTIVE: To estimate the effect of soybean diet on serum level of thyroid hormone, its metabolites and thyrotropin (TSH) during aging in rats. METHODS: Male Donryu rats were fed laboratory chow containing 40 (Group A) or 10 volume percent (Group B) soybean protein, while controls (Group C) received regular laboratory chow. Groups of 10 animals of each groups were sacrificed by decapitation at the age of 12, 18, 24 and 30 months. Serum total thyroxine (T4), free thyroxine (FT4), 3,5,3'-triiodothyronine (T3), 3,3',5'-triiodothyronine (rT3) and 3,3'-diiodothyronine (3,3'-T2) and TSH concentrations were measured by specific radioimmunoassays. RESULTS: In Group A the level of T3 decreased significantly at from the age of 18 months, while in other groups such decrease was found only from the age of 24 months. Such changes were closely resembled by these in the level of 3,3'-T2, while inverse changes were observed in the level of rT3 which was increased in Group A from the age of 18 months and in the other groups from the age of 24 months. Serum T4 and FT4 level was decreased in all groups at the age of 30 months and no changes were observed in the level of TSH. CONCLUSIONS: The findings suggest that the level of T4, FT4 and T3 with its metabolite 3,3'-T2 stepwise decreased with aging, while that of rT3 showed inversely and increase. These changes were influenced by the content of soybean protein in the diet, the most rapid changes being found in the group with the high content of such protein.     

3,3',5'-Triiodothyronine (reverse T3) and 3,3',5-triiodothyronine (T3) in fetal and adult sheep: studies of metabolic clearance rates, production rates, serum binding, and thyroidal content relative to thyroxine.

To examine the mechanism(s) responsible for high serum concentration of 3,3',5'-triiodothyronine (reverse T3, rT3) and low serum concentration of 3,3',5-triiodothyronine (T3) in the fetus, we studied metabolic clearance rates (MCR) and production rates (PR) of rT3, T3, and thyroxine (T4) in adult nonpregnant sheep and sheep fetuses in utero. The mean fetal MCR-rT3 was significantly lower than that in adult sheep, and the mean fetal PR-rT3 significantly higher. The mean fetal MCR-T3 was higher than, and the mean fetal PR-T3 similar to that in adult sheep. The mean fetal MCR-T4 and PR-T4 were both significantly higher than the corresponding values in adult sheep. The ratios of mean PR-rT3 to PR-T4 (rT3/T4) were similar in fetal and adult sheep. However, the ratio of mean PR-T3 to PR-T4 (T3/T4) in the fetal sheep was much lower than that in the adult sheep. The low fetal MCR-rT3 was not attributable to high serum binding of rT3. On the basis of the thyroidal content and kinetics of iodothyronines, it was estimated that whereas thyroidal secretion may account for nearly all of serum T3 (or PR-T3) in the fetus and about 50% of serum T3 in adults, it accounts for only about 3% of the serum rT3 (or PR-rT3) in both fetal and adult sheep. The results suggest a) that elevated serum rT3 in the fetus is due to its decreased clearance and increased production by mono-deiodination of T4, and b) that low serum T3 in the fetus is due to its increased clearance and decreased production by mono-deiodination of T4. In addition, on the basis of discordant changes in the production of T3 and rT3 from T4, it appears that there may exist two separate, apparently specific, iodothyronine deiodinating activities--one cleaving the iodine atom at the 5'-position and the other acting in the iodine atom at the 5-position of the T4 molecule; 5'-iodothyronine deiodinating activity is apparently reduced in the fetus.     

Deiodination of iodothyronine sulfamates by type I iodothyronine deiodinase of rat liver

The substrate behavior of synthetic N-sulfonated iodothyronines (iodothyronine sulfamates, TiNS) for the type I deiodinase was compared with that of the naturally occurring 4'-O-sulfonated iodothyronines (iodothyronine sulfates, TiS), which have been shown to be deiodinated 40-200 times more efficiently than the native iodothyronines. Deiodination was studied in incubations of rat liver microsomes with unlabeled or 3' (5')-125I-labeled T4NS, rT3NS, T3NS, and 3,3'-T2NS at 37 C and pH 7.2 in the presence of 5 mM dithiothreitol. Reaction products were analyzed by RIA or Sephadex LH-20 and HPLC. Kinetic studies were performed under initial reaction rate conditions to determine the apparent Michaelis Menten (Km) constants and maximum velocity values. In contrast to T4S, which is converted only by inner ring deiodination (IRD), T4NS underwent both IRD and outer ring deiodination (ORD), similar to T4, but more rapidly. At 10 nM T4NS substrate, T3NS was the major product observed, while no rT3NS accumulated due to its rapid conversion to 3,3'-T2NS. At least one third of the 3,3'-T2NS was converted by IRD, unlike 3,3'-T2 which is a pure ORD substrate. The type I deiodination efficiencies of T4NS IRD and ORD were 17-fold higher than with T4, mainly due to approximately 32-fold lower apparent Km values. Deiodination of rT3, the preferred type I substrate, was not improved by sulfamation. T3NS and 3,3'-T2NS were deiodinated 4-10 times more efficiently than T3 and 3,3'-T2, respectively, due to 2- to 4-fold decreases in apparent Km values with a concomitant doubling of maximum velocity values. N-Sulfonation stimulates type I deiodination to a similar extent as other side-chain modifications that eliminate the positive charge of the nitrogen (e.g. iodothyroacetic acids). However, the effects are less dramatic than those induced by 4'-sulfation with respect to both efficiency and specificity of the catalytic process.     

3'-5'-Diiodothyronine in health and disease: studies by a radioimmunoassay.

A simple, reproducible, and highly specific RIA has been developed for measurement of 3',5'-diiodothyronine ((3',5'-T2) in unextracted serum. Interference in binding of radioactive 3',5'-T2 to anti-3',5'-T2 by serum proteins was minimized by using 0.4 M phosphate buffer (pH 6.2) and merthiolate. The detection threshold of the RIA was 2.5 ng/100 ml. Recovery of nonradioactive 3',5'-T2 added to serum averaged 99%. T4, T3, and rT3 cross-reacted with 3',5'-T2-binding sites on anti-3',5'-T2 antibody only to the extent of 0.0025, less than 0.0004, and 0.22%, respectively. 3'-Monoiodothyronine cross-reacted 1.7%. Serum 3',5'-T2 concentrations were (mean +/- SD) 6.4 +/- 2.4 ng/100 ml in 53 normal subjects, 4.2 +/- 3.5 ng/100 ml in 7 hypothyroid patients, 14.9 +/- 7.7 ng/ml in 25 patients with hepatic cirrhosis, and 14.3 +/- 5.3 ng/100 ml in 31 newborns' cord blood sera. The values in each of the latter four groups were significantly different from normal. The mean serum 3',5'-T2 concentration of 7.7 +/- 2.5 ng/ml in eight subjects in the third trimester of pregnancy did not differ significantly from normal at a time when serum T4 and T3 were clearly elevated. Oral administration of 300 microgram rT3 to 9 normal subjects led to a mean maximal increase in serum 3',5'-T2 concentration of 45% at 1 h. Total fasting in 3 obese subjects was associated with a significant increase in serum 3',5'-T2 from 8.6 to 16.3 ng/100 ml at 6-8 days; serum rT3 increased similarly, while serum T3 decreased and T4 did not change. Administration of dexamethasone (2 mg also associated with nearly parallel increases in serum 3',5'-T2 and rT3 and a decrease in serum T3. 3',5'-T2 concentrations were also measured in amniotic fluids at different stages of gestation; the mean value of 15.2 ng/100 ml at 15-20 weeks gestation was significantly higher than that of 5.8 ng/ml at 33-40 weeks gestation. Pronase hydrolysates of 9 autopsy specimens of normal thyroid glands contained (mean +/- SD) 350 +/- 144 microgram T4 and 0.24 +/- 0.15 microgram 3',5'-T2/g wet wt. On the basis of these data and those available for MCRs of 3',5'-T2 and T4, it was estimated that thyroidal secretion contributes less than 1% of 3',5'-T2 measured in serum of normal man. The various data suggest that: 1) 3',5'-T2 is a normal component of human serum; 2) almost all 3',5'-T2 in human serum derives from extrathyroidal sources; and 3) changes in serum 3',5'-2 generally parallel those in rT3.     

The relative distribution of thyroxine, triiodothyronine and 3,3',5'-(reverse)-triiodothyronine in various fractions of thyroglobulin

Thyroglobulin fractions rich and poor in new thyroglobulin were separated by means of DEAE-cellulose chromatography of dog thyroid extracts and by zonal ultracentrifugation in a sucrose gradient of guinea pig thyroid extract incubated at low temperature. The distrubtion of thyroxine, triiodothyronine and 3,3',5'-(reverse)-triidothyronine in hydrolysates of the different fractions was estimated by radioimmunoassays. Following DEAE-cellulose chromatography there was a small but statistically significant increase in T4/T3 ratio in thyroglobulin fractions eluted at high ionic strength--that is fractions relatively rich in stable iodine but poor in fresh thyroglobulin. There was no differences in the T4/rT3 ratios between the different fractions. The ratios between iodothyronines were almost identical in the various thyroglobulin fractions following zonal ultracentrifugation in a sucrose gradient of cold treated guinea pig thyroid extract. These findings lend no support to the possibility that a relatively high content of triiodothyronines in freshly synthesized thyroglobulin modulates the thyroid secretion towards a preferential secretion of triiodothyronine and 3,3',5'-(reverse)-triidothyronine at the expense of the secretion of thyroxine.     

Comparison of inhibitory effects of 3,5,3'-triiodothyronine (T3), thyroxine (T4), 3,3,',5'-triiodothyronine (rT3), and 3,3'-diiodothyronine (T2) on thyrotropin-releasing hormone-induced release of thyrotropin in the rat in vitro.

In order to compare, in vitro, the TSH suppressive effects of iodothyronines, rat pituitary quarters were first preincubated with T4, T3, rT3, or 3,3'-diiodothyronine (T2) in Gey and Gey buffer containing 1% bovine serum albumin for 2 h at 37 C and then incubated at 37 C for 1 h with the iodothyronine under study and TRH. TSH released into the medium during incubation was compared to that released by control pituitary fragments, which were not exposed to iodothyronines. All four iodothyronines (T3, T4, rT3, and T2) were able to significantly inhibit the TRH-induced release of TSH from pituitary fragments in a dose range of 0.015-2.2 microgram/ml. However, much larger doses of sodium iodide (1.25 mg/ml) and diiodotyrosine (10 and 30 microgram/ml) had no significant effect on the release of TSH. Among T3, rT3, and T4, T3 was the most potent and rT3 was the least potent. The relative potency of T3:T4:rT3 appeared to be approximately 100:12:1 when estimated from the lowest doses that caused significant inhibition of TRH-induced release of TSH, and approximately 100:6:0.5 when estimated from the doses that caused 50% inhibition of TSH release; the TSH inhibiting potency of T2 was similar to that of rT3. The activity of T4 could not be explained entirely on the basis of contamination of T4 with T3 or by in vitro conversion of T4 to T3. Similarly, the available data suggested that rT3 and T2 possess some, albeit modest, intrinsic TSH-Suppressive activity. TSH-inhibiting activities of T3, T4, and rT3 were also studied using pituitary fragments from starved and iodine-deficient rats. There was no evidence of a change in the sensitivity of the thyrotroph to either T3 or T4 in starvation. Similarly, comparison of the responses to several doses of rT3 did not indicate any significant abnormality in the sensitivity of the thyrotroph to rT3 in starvation or iodine deficiency. However, comparison of the TSH-suppressive effects of T4 in the iodine-deficient and normal rat indicated a significant increase in the sensitivity of the thyrotroph to T4 in iodine deficiency. A similar trend was also evident in the effect of T3 in iodine deficiency, but it fell short of statistical significance.     

Opposite effects of dexamethasone on serum concentrations of 3,3',5'-triiodothyronine (reverse T3) and 3,3'5-triiodothyronine (T3).

Dexamethasone, 2 mg every 6 hours for 4 doses, was given to 4 hypothyroid patients receiving treatment with synthetic thyroxine (T4) and to 8 untreated hyperthyroid patients with Graves' disease, and serum concentrations of thyroid hormones were measured by radioimmunoassays. Serum concentration of 3,3'5'-triiodothyronine (reverse T3, rT3) increased appreciably within 8 hours after the first dose of dexamethasone, was maximum at 24-32 hours after beginning dexamethasone, and remained elevated for about 24 hours after discontinuing the steroid. The mean baseline serum rT3 was 58 ng/per 100 ml in treated hypothyroid patients and 119 ng per 100 ml in patients with Graves' disease; the corresponding maximal post-dexamethasone serum rT3 values were 87 and 170 serum concentration of 3,3',5-triiodothyronine (T3) decreased. The decrease in serum T3 was significant at about 24 hours after beginning dexamethasone and was maximal at about 30 hours in both groups of cases under study. The decrease in serum T3 persisted in treated hypothyroid cases for about 24-48 hours and in Graves' disease cases as long as studied, at least 5 days after discontinuing hexamethasone. The changes in serum rT3 and T3 could not be attributed to the effect of dexamethasone on serum protein binding of the iodothyronines because the dialyzable fractions of rT3 and T3 following steroid administration were not different from those before it. Serum T4 did not change appreciably in treated hypothyroid cases, but decreased in Graves' disease cases from a mean baseline value of 23.5 mug per 100 ml to 18.4 mug per 100 ml 3 days after beginning dexamethasone. In addition, 3 hyperthyroid cases were studied before, during, and after administration of dexamethasone, 2 mg every 6 h for 5 days. Serum rT3 increased again as noted above and the increase persisted until about 24 hours after the last dose of the steroid. Serum T3 decreased considerably and remained decreased as long as studied, at least 4 days after discontinuing the steroid. Serum T4 decreased appreciably in 2 of the 3 cases studied. The data suggest that 1) conversion of T4 to T3 and to rT3 may occur via two distinct pathways in the metabolism of T4; 2) the changes in serum rT3 and T3 observed in our study may be due in part at least to a steroid-induced 'shift' in the metabolism of T4 whereby conversion of T4 to T3 is diminished and that to rT3 is enhanced; 3) in addition to the effect on peripheral metabolism of T4, steroids appear to reduce the circulating thyroid hormones in Graves' disease by another mechanism, probably by reduction in thyroid secretion.