Fasting decreases thyrotropin responsiveness to thyrotropin-releasing hormone: a potential cause of misinterpretation of thyroid function tests in the critically ill

We have previously reported that caloric deprivation inhibits peripheral T4 metabolism and blunts the TSH response to TRH in euthyroid obese subjects. To determine whether these phenomena also occur in hypothyroid subjects, T4, T3, rT3, and the TSH response to TRH were measured initially and after a 60-h fast in seven hypothyroid patients. Short term fasting caused a 29% decrement in the maximum serum TSH increment and a 32% decrement in the integrated TSH response to TRH (P less than 0.01). In two subjects with mild hypothyroidism, basal TSH as well as the TSH response to TRH were reduced to levels within the normal range. Specifically, basal TSH values decreased from 7.6 to 3.5 microU/ml and from 11 to 4.1 microU/ml. In the seven subjects, mean serum T3 decreased significantly from 88 to 60 ng/dl, (P less than 0.05) and rT3, initially undetectable in six of seven subjects, rose to detectable or low normal values in four of seven subjects, serum T4 remained at 2.7 micrograms/dl during both study periods. We conclude that 1) fasting induces changes in both peripheral thyroid hormone metabolism and the hypothalamic-pituitary axis in hypothyroid individuals which are qualitatively similar to those that occur in euthyroid subjects; and 2) in certain hypothyroid subjects, fasting alone can decrease basal TSH values to within the normal range. If these data can be extrapolated to critically ill subjects whose caloric intake may be diminished, they suggest that basal TSH concentrations in moderately and severely hypothyroid critically ill subjects will accurately reflect the biochemically hypothyroid state. However, mild degrees of hypothyroidism in critically ill subjects might be overlooked due to the lowering effect of fasting or poor caloric intake alone on basal TSH concentrations.     

Alterations in circulating thyroid hormones and thyrotropin in euthyroid patients with Graves' disease after total thyroidectomy: comparison between responders and nonresponders to TSH-releasing hormone

Eleven euthyroid patients with severe exophthalmos of Graves' disease who had been treated with antithyroidal drugs for one to three years prior to total thyroidectomy were studied. All patients were clinically and biochemically euthyroid at the time of operation. According to their responses of TSH to TRH prior to operation, the patients were divided into two groups: (1) five responders and (2) six nonresponders. In group 1, serum TSH levels increased significantly on the third day after thyroidectomy (from 1.5 +/- 0.3 to 8.6 +/- 1.4 microU/mL: P less than 0.05); serum T4 concentrations decreased significantly and were in the hypothyroid range by the third day. In group 2, serum TSH levels rose from 0.5 +/- 0.01 to 3.2 +/- 0.5 microU/ml (P less than 0.05) on the ninth postoperative day; serum T4 concentrations decreased on the third day after operation but did not attain hypothyroid levels until the 12th day. Thus after total thyroidectomy the following are concluded: (1) serum TSH levels even in treated euthyroid patients with Graves' disease, rose more gradually in TRH-nonresponders in comparison with TRH responders; (2) the time when serum TSH elevation occurs is dependent upon serum concentrations of thyroid hormones (serum T3 and T4).     

Thyrotropin secretion in thyrotoxic and thyroxine-treated patients: assessment by a sensitive immunoenzymometric assay

A new TSH immunoenzymometric assay was found to be capable of discriminating between the serum TSH values of normal subjects [2.28 +/- 1.02 (+/-SD); range, 0.6-6.5 microU/ml] and those of clinically euthyroid, antithyroid drug-treated (n = 22) or clinically thyrotoxic (n = 34) patients. While a wide spectrum of basal TSH values was found in the antithyroid drug group [ranging from undetectable (less than 0.05 microU/ml: 57%) to 17.9 microU/ml], all clinically thyrotoxic patients had undetectable values. In 33 patients receiving chronic oral T4 therapy for treatment of goiter (n = 15) or thyroid cancer (n = 18), 48% (6 of 33) had undetectable basal TSH levels and no TSH response to TRH stimulation. Detectable TSH levels were found in 42% (14 of 33), and TRH responsiveness was found in 52% (17 of 33). The TSH response to TRH stimulation was less than 2.0 microU/ml in 7 patients. Serum free T4 index, free T3 index, and free T4 levels and oral T4 dosage were inferior predictors of TRH responsiveness compared to the basal TSH value. No patient receiving more than 0.2 mg T4 daily or having a free T4 index above 18, a free T3 index above 205 or a free T4 level above 3.0 ng/dl had a TSH response to TRH. Seventy-six percent (16 of 21) of the patients, when reevaluated 1-6 weeks after increased oral T4 dosage, had a significant reduction in their serum thyroglobulin level. This was true of both patients with initially detectable (11 of 14) as well as undetectable (5 of 7) basal serum TSH levels. These findings support the concept that subnormal and, for that matter, as yet undetectable levels of circulating TSH may exert stimulatory effects on thyroid tissue.     

Different rates of thyrotropin suppression after total body scan in patients with thyroid cancer: effect of an optimal saturation regimen with thyroxine or triiodothyronine

The rate of TSH suppression in patients with differentiated thyroid cancer, when therapy is re-started after total body scan, was investigated adopting an optimal saturation regimen, either with T4 or with T3. The first group of 6 patients received T4 as follows: from day 1 to 7 = 22, 11, 6, 4, 3.5, 3.2, 3.2 micrograms/day/kg body weight (b w) and continued with 3.2; the second group of 8 patients received T3 as follows: 2.4, 1.8, 1.4, 1.2, 1.1, 1.1 micrograms/day/kg BW and continued with 1.1. At time 0, TSH levels were high in all patients (range 80-180 microU/ml); T3 and T4 levels were below the limit of detectability. After the beginning of the therapy, the decrease of TSH levels and the inhibition of TSH response to TRH occurred faster in patients taking T3 than in patients taking T4. In the former, at day 7, mean basal TSH level was 1.9 +/- 0.5 microU/ml and 30 min after 200 micrograms TRH iv mean TSH level was 9.9 +/- 4.4 microU/ml; at day 10 they were 1.4 +/- 0.5 and 2.7 +/- 0.8 microU/ml respectively. In the latter, at day 7, mean basal TSH level was 4.6 +/- 3.9 microU/ml and 30 min after TRH mean TSH level was 42.2 +/- 34.2 microU/ml. Only at day 20 they were 0.8 +/- 0.2 and 1.2 +/- 0.9 microU/ml respectively. In patients taking T3 by saturation regimen, serum levels of T3 rose rapidly to supranormal values (at day 3, mean serum T3 level was 297 +/- 62 ng/100 ml), reached a peak at day 5 (340 +/- 62 ng/100 ml) and decreased thereafter, always remaining however above normal limits.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of phenobarbital on hypothalamic-pituitary-thyroid axis in the rat

It has been reported that phenobarbital (PB) increases the peripheral clearance of T4 and T3 and decreases serum T4 and T3 concentrations in the rat, but serum TSH remains unchanged. To explore a possible direct effect of PB on TSH secretion at the hypothalamic-pituitary level, adult male rats were given PB 100 mg/kg or vehicle IP for 10 days. No difference in their thyroid weights was observed. In the PB-treated group serum T4 was decreased (PB, 3 +/- 0.2 micrograms/dl vs. control, 3.8 +/- 0.1 micrograms/dl, mean +/- SE, p less than .002), as was serum T3 (PB, 51 +/- 6 ng/dl vs. control, 70 +/- 5 ng/dl, p less than .05), but serum TSH remained unchanged. Pituitary TSH and hypothalamic TRH contents also were unchanged. Further studies were carried out similarly in the thyroidectomized hypothyroid rat to eliminate the effect of PB on serum T4 and T3 levels. PB or vehicle were started two days after thyroidectomy. By postoperative day 12, TSH levels in the PB-treated rats were lower than in the controls (PB, 697 +/- 62 microU/ml vs. control, 891 +/- 53 microU/ml, p less than .05). Pituitary TSH and hypothalamic TRH contents again were similar in both groups. When TRH (500 ng/kg body weight, IV) was given, the increment in serum TSH at 10 minutes was significantly lower in the PB group (PB, 53 +/- 26 microU/ml vs. control, 131 +/- 18 microU/ml, p less than .05).(ABSTRACT TRUNCATED AT 250 WORDS)     

IODIDE ADMINISTRATION ENHANCES THYROTROPHIN RESPONSIVENESS TO THYROTROPHIN-RELEASING HORMONE DURING FASTING: EVIDENCE FOR NORMAL PITUITARY FEEDBACK REGULATION

Short-term fasting in humans is associated with diminished delta TSH to TRH. The purposes of the present study were to reassess basal TSH levels and TRH responsiveness during fasting utilizing a sensitive radioimmunoassay (RIA: sensitivity 0.3 microU/ml; normal range 0.66-2.98 microU/ml) and to determine if normal feedback regulation is maintained during the fasting state. Eight control subjects (C) and six iodide-treated (I) subjects (262 mg/d) were studied in the fed state and on day 10 of fasting. T3, T4, and TSH were measured by RIA, and free T4 and free T3 by equilibrium dialysis. Basal serum TSH levels in the control group were 2.0 +/- 0.3 microU/ml (mean +/- SEM) in the fed state and increased to 14.7 +/- 3.5 microU/ml 20 min after TRH administration. The fasting basal TSH level of 1.6 +/- 0.3 microU/ml was significantly decreased (P less than 0.01) compared to control, as was the level of 8.8 +/- 2.3 microU/ml (P less than 0.01) obtained 20 min after TRH. In the iodide-treated group the basal TSH level was 1.4 +/- 0.2 microU/ml during feeding which increased (P less than 0.025) to 2.9 +/- 0.7 microU/ml during fasting; the TSH value 20 min after TRH was 12.6 +/- 2.5 microU/ml while feeding and 17.3 +/- 2.9 microU/ml while fasting. Free and total T3 decreased during fasting in both groups. Total T4 was unchanged between the fed and fasted periods in the two groups.(ABSTRACT TRUNCATED AT 250 WORDS)     

The influence of long-term low dose thyrotropin-releasing hormone infusions on serum thyrotropin and prolactin concentrations in man

The purpose of the present study was to evaluate in man the relative thyrotroph and lactotroph response to a 48-h low dose constant TRH infusion. Before, during, and after the 75 ng/min TRH constant infusion, serum samples were obtained every 4 h in six euthyroid ambulating male subjects for measurements of TSH, PRL, T4, and T3. The TSH response, employing a specific and sensitive human TSH RIA, demonstrated a significant rise from the mean basal pre-TRH value of 2.35 +/- 0.64 microU/ml (+/- SEM) to 3.68 +/- 0.80 (P < 0.005) during the TRH infusion; this value fell below the basal level to 1.79 +/- 0.47 (P < 0.05) post infusion. Serum T4 values were increased above basal both during (P < 0.025) and after (P < 0.025) TRH infusion, whereas serum T3 values were not significantly changed throughout the entire study period. The daily TSH nocturnal surge was augmented in both absolute and relative terms during the first 24 h or the TRH infusion, unchanged during the second 24 h of infusion, and inhibited during the first postinfusion day. Other than a minimal increase in serum PRL during the first few hours of the infusion, no significant alteration in the mean basal concentration or circadian pattern of PRL secretion was evident during or after the low dose TRH infusion. These findings would indicate that 1) near-physiological stimulation of the pituitary with TRH produces a greater stimulation of TSH release than of PRL release and 2) the factor or factors producing the circadian TSH surge may not be mediated through fluctuations in endogenous TRH.     

Effects of oestradiol benzoate on the pituitary secretion and peripheral kinetics of thyrotrophin in the rat

Previous works from this laboratory have demonstrated that oestradiol benzoate (EB) in euthyroid male and female rats induced a significant decrease in the pituitary content of TSH while serum levels of this hormone remained normal. The present work studied the effects of EB (25 micrograms/100 g body weight, during 9 days) on the peripheral metabolism of [125I]rTSH and on the pituitary and plasma concentration of TSH in euthyroid and hypothyroid rats. No significant variations were observed in [125I]rTSH kinetics of EB-treated euthyroid rats vs untreated controls: fractional turnover rate 2.8 +/- 0.2 vs 3.0 +/- 0.3%/min, distribution space 6.5 +/- 0.4 vs 6.8 +/- 0.5 ml/100 g body weight, disposal rate 18.4 +/- 2.4 vs 18.1 +/- 1.9 microU/100 g/min and extrapituitary pool 645 +/- 42 vs 614 +/- 43 microU/100 g body weight. Similarly, in hypothyroid rats oestrogens induced no changes in TSH kinetics except for an increase in distribution space (P less than 0.025). However, oestrogens decreased the pituitary pool of TSH (P less than 0.001) in both euthyroid and hypothyroid rats and increased the plasma TSH in hypothyroid animals (P less than 0.01), all vs their respective controls. Neither hypothyroid group had detectable plasma levels of T4 and T3. In summary: 1) the marked decrease of pituitary TSH with normal plasma TSH induced by EB appears unrelated to the peripheral metabolism of TSH, 2) the results from hypothyroid rats suggest that EB stimulates the release of TSH from the pituitary gland.     

Effect of prolonged oral administration of TRH on plasma levels of thyrotrophin and prolactin in normal individuals and in patients with primary hypothyroidism.

Forty mg TRH/day was given orally for 3 weeks to 10 euthyroid women and 10 women with primary hypothyroidism on low replacement doses of thyroxine. Once weekly oral TRH was replaced by an iv TRH-test (0.4 mg) with measurement of serum concentration of TSH, prolactin (PRL), thyroxine (T4), triiodothyronine (T3) and cholesterol. In the normal group, mean serum T4 concentration increased after one week and remained elevated. Serum TSH concentration showed a slight tendency to decline. Maximal rise in TSH concentration after iv TRH (deltaTSH) fell from a mean of 4.0 ng/ml to 1.4 ng/ml within one week and stayed low. T3, cholesterol, PRL and deltaprl were normal and unchanged throughout. In the hypothyroid group T4, T3, cholesterol, PRL and deltaPRL were not influenced by the TRH administration. In 2 patients (with the highest serum T4 concentrations) serum TSH concentration was normal and resistant to iv TRH. Of the 8 patients with elevated TSH, basal level and deltaTSH did not change in 2 (with subnormal T4 levels and the highest TSH levels). In the other 6 (with intermediate T4 levels) basal TSH fell from a mean of 10.1 ng/ml to 4.2 ng/ml, and deltaTSH from 10.0 ng/ml to 3.3 ng/ml after three weeks. It is concluded that in addition to feed-back effect of thyroid hormones, the pituitary response to long-term administration of TRH is determined by other factors. Among these may be reduced pituitary TRH receptor capacity and the activity of the TSH producing cells.     

Failure of 3,3'-diiodothyronine administration to alter TSH and prolactin responses to TRH stimulation.

In order to determine whether elevations in serum 3,3'-diiodothyronine (3,3'T2) concentrations influence the hypothalamic-pituitary--thyroid axis, thyrotropin (TSH) and prolactin responses to thyrotropin-releasing hormone (TRH) were assessed in five patients both prior to and during 3,3'T2 administration. Mean (+/- SE) peak TSH responses to TRH were 168 +/- 64 microU/ml during 3,3'T2 administration and 168 +/- 65 muU/ml during 3,3'T2 administration. Mean basal and peak prolactin concentrations after TRH were 6 +/- 3 ng/ml and 54 +/- 26 ng/ml, whereas during 3,3'T2 administration the basal and peak prolactin levels were 6 +/- 2 ng/ml and 55 +/- 28 ng/ml, respectively. Hypothyroid rats administered triiodothyronine (10 migrogram b.i.d.) for 5 days had a mean TSH response to TRH stimulation of 0.051 +/- 0.003 mU/ml, whereas rats to whom saline or 3,3'T2 (50 microgram b.i.d.) had been given for the same time interval had mean TRH-induced TSH responses of 1.127 +/- 0.179 mU/ml and 1.324 +/- 0.286 mU/ml, respectively. None of the TSH or prolactin responses to TRH, in either human or rat studies, were apparently altered by 3,3'T2. These observations suggest that elevation of serum 3,3'T2 levels are not associated with alterations in the hypothalamic--pituitary--thyroid axis in the experimental systems employed.     

Serum 3,5,3'-triiodothyronine, thyroxine, and thyrotropin in hypothyroid infants with congenital goiter and the response to iodine.

Iodine deficiency in adults caused preferred synthesis of T3; this observation has not been reported in iodine-deficient hypothyroid newborns. Serum total T4, total T3, and TSH have been determined in nine full term newborns with congenital hypothyroid goiter before and after cutaneous application of iodine. The mothers of these infants had untreated euthyroid goiter and lived during pregnancy in the area of G?ttingen, West Germany, known as an iodine-deficient region. Mean total T4 in the newborns was 6.3 +/- 1.6 (mean +/- SD) micrograms/dl compared to 16.6 +/- 3.4 micrograms/dl in normal newborns at 3-4 days of age. Mean T3 in the goitrous newborns was 2.74 +/- 0.66 ng/ml compared to 1.58 +/-0.41 ng/ml in the control group of the same age. Serum TSH remained elevated during the first week of life, with a concentration of 40.9 +/- 28.7 microU/ml (control group, 4.16 +/- 1.43 microU/ml). The cutaneous application of iodine resulted in rapid disappearance of goiter and normalization of T4 and TSH within 5 days. After 30 days of iodine treatment, T3 decreased slowly but remained elevated (2.0 +/- 0.42 ng/ml vs. 1.67 +/- 0.36 ng/ml in the control group). The present findings confirm preferential T3 secretion in newborns with hypothyroid goiter. The goiter is thought to be caused by intrauterine iodine deficiency, because hypothyroid values of T4 and TSH normalized during iodine treatment. General iodine prophylaxis of the population is recommended.     

Decreased adrenergic sensitivity in patients with hypothyroidism

Cardiovascular sensitivity to catecholamines was assessed in 15 patients with hypothyroidism (mean [+/- SEM] thyroxine [T4] index 2.7 +/- 0.5 micrograms/100 ml, thyroid stimulating hormone [TSH] 136.9 +/- 48.3 microU/ml), aged 45 +/- 4 years and in 8 healthy control subjects. The study was repeated in 10 patients with hypothyroidism 4.0 +/- 0.5 months after thyroid replacement therapy (T4 index 9.9 +/- 2.1 micrograms/100 ml, TSH 3.5 +/- 1.3 microU/ml). In addition, basal, average and maximal heart rates were measured using 24 h ambulatory electrocardiographic (ECG) monitoring, and plasma levels of epinephrine and norepinephrine were determined before and after thyroid replacement. Heart rate increased less after bolus injection of 0.8, 1.6 and 3.2 micrograms of isoproterenol in the hypothyroid (10 +/- 2, 15 +/- 2 and 21 +/- 4 beats/min, respectively) than in the euthyroid (16 +/- 3, 22 +/- 3 and 30 +/- 4 beats/min, respectively) state (p less than 0.05). Control subjects reacted similarly to patients receiving thyroid replacement. Basal heart rate (64 +/- 3 versus 68 +/- 3 beats/min, p less than 0.05) and maximal heart rate (116 +/- 5 versus 133 +/- 5 beats/min, p less than 0.05) were lower on 24 h ambulatory ECG monitoring in the hypothyroid than euthyroid state despite higher basal plasma norepinephrine levels (394 +/- 45 versus 315 +/- 45 pg/ml, p less than 0.05). Thus, patients with hypothyroidism display a decreased cardiac chronotropic response to beta-adrenergic stimulation. This may contribute in part to the decreased basal and maximal daily heart rates seen in patients with hypothyroidism, which occurs despite elevated plasma norepinephrine levels.     

Thyroglobulin release after graded endogenous thyrotropin stimulation in man: lack of correlation with thyroid hormone response

The serum thyroglobulin (Tg), T3, and T4 responses to graded endogenous TSH stimulation were examined in 30 normal subjects for up to 96 h after TRH administration. Increasing TSH rises were elicited by TRH administration as follows: 1) 500 micrograms iv as a single bolus in 10 subjects [mean peak serum TSH, 14.3 +/- 1.8 (SE) microU/ml]; 2) 1000 micrograms infused iv in 2 h in 10 subjects (mean peak TSH, 25.5 +/- 2.6 microU/ml); 3) 40 mg orally in 10 subjects (mean peak TSH, 27.5 +/- 3.0 microU/ml, with a delayed and more prolonged rise). Nine subjects received saline and were used as controls. A significant serum T3 and T4 rise followed the TSH increase in all subjects, and the mean peak value was always reached 4 h after TRH. In contrast, a significant serum Tg increase occurred only in 3, 6, and 9 subjects after 500 micrograms, 1000 micrograms, and 40 mg TRH, respectively. In addition, the time of the Tg peak and its duration was extremely variable but it was always delayed in respect to serum T3 and T4 peaks, occurring 6 to 72 h after TRH administration. No correlation was found between serum Tg and T3 or T4 increases after TRH in any of the three groups. These studies indicate that a significant Tg release in man usually occurs only after intense and prolonged TSH stimulation of the thyroid. In addition, the Tg increase is delayed in respect to the thyroid hormone increase and it is not correlated with them.     

TRH test as an index of suppression compared with the thyroid radioiodine uptake in euthyroid goitrous patients treated with thyroxine.

To determine an index of adequate suppression of pituitary TSH secretion in euthyroid goitrous patients treated with sodium levothyroxine (T4), TSH responses to 500 micrograms TRH given iv were compared with thyroid 24-h radioiodine uptakes during therapy with T4 in 12 euthyroid goitrous patients. The patients received sequentially 100, 150, 200, 250, and 300 micrograms T4 with the doses increased at 4-6 week intervals. The mean dose of T4 that reduced the peak TSH response to TSH to the lower limit of normal (TSH = 5 microU/ml) was 130 micrograms; the mean T4 dose that suppressed the TSH response to one-half the lower limit of normal (TSH = 2.5 microU/ml) was 165 micrograms. The mean T4 dose that nearly obliterated the TSH response was 200 micrograms; this degree of suppression occurred with doses of 100-300 micrograms T4 in individual patients. Suppression of thyroid uptake correlated closely with suppression of the TSH response to TRH. The goiter diminished in size significantly in 6 of the 12 patients during the 6 months of observation adn did not enlarge in any patient. The data indicate that suppression of the TSH response to TRH is a convenient technique to assess the adequacy of suppressive therapy of goiter.     

Decreased growth hormone (GH) response to oral clonidine in endemic cretinism: effect of L-T3 therapy

As GH secretion is dependent upon thyroid hormone availability, the GH responses to clonidine (150 micrograms/m2) and the TSH and PRL response to TRH were studied in eight endemic (EC) cretins (3 hypothyroid, 5 with a low thyroid reserve) before and after 4 days of 100 micrograms of L-T3. Five normal controls (N) were also treated in similar conditions. Both groups presented a marked increase in serum T3 after therapy (N = 515 +/- 89 ng/dl; EC = 647 +/- 149 ng/dl) followed by a decrease in basal and peak TSH response to TRH. However, in the EC patients an increase in serum T4 levels and in basal PRL and peak PRL response to TRH after L-T3 therapy was observed. One hypothyroid EC had a markedly elevated PRL peak response to TRH (330 ng/dl). There were no significant changes in basal or peak GH values to treatment with L-T3 in normal subjects. In the EC group the mean basal plasma GH (2.3 +/- 1.9 ng/ml) significantly rose to 8.8 +/- 3.2 ng/ml and the mean peak response to clonidine (12.7 +/- 7.7 ng/ml) increased to 36.9 +/- 3.1 ng/ml after L-T3. Plasma SM-C levels significantly increased in N from 1.79 +/- 0.50 U/ml to 2.42 +/- 0.40 U/ml after L-T3 (p less than 0.01) and this latter value was significantly higher (p less than 0.05) than mean Sm-C levels attained after L-T3 in the EC group (respectively: 1.14 +/- 0.59 and 1.78 +/- 0.68 U/ml). These data indicate that in EC the impaired GH response to a central nervous system mediated stimulus, the relatively low plasma Sm-C concentrations, and the presence of clinical or subclinical hypothyroidism may contribute to the severity of growth retardation present in this syndrome.     

Is the thyrotropin-releasing hormone test necessary in the diagnosis of central hypothyroidism in children

To determine the value of the TRH test, we analyzed the unstimulated serum T(4) and TSH concentrations in 54 children with central hypothyroidism. A TRH test was performed in 30 patients. Midline brain defects (septo-optic dysplasia, 28; holoprosencephaly, 2) and combined pituitary hormone deficiencies were present in 30 and 52 patients, respectively. The mean serum free T(4), total T(4), and basal TSH concentrations were 0.6 ng/dl, 4.0 microg/dl, and 2.8 microU/ml, respectively. Five patients demonstrated elevated basal serum TSH concentrations. A normal TRH test [increase (delta) in TSH, 4.5-17.8], based on data from 30 controls, was documented in 23.3% of patients. Brisk (deltaTSH, >17.8), absent/blunted (deltaTSH, <4.5), and delayed responses were documented in 16.7%, 30%, and 30% of patients, respectively. The mean age at diagnosis was 2.8 yr, with 8 patients evolving into TSH deficiency. It was not possible to differentiate patients as having pituitary or hypothalamic disease based solely on the TRH test results. Patients with septo-optic dysplasia were diagnosed earlier and had elevated basal serum TSH and PRL concentrations, diabetes insipidus, and evolving disease. Although full pituitary function assessment is mandatory to identify combined pituitary hormone deficiencies, a TRH test is not essential, and the diagnosis should be made by serial T(4) measurements.     

A radioimmunoassay for 3,3',5'-L-triiodothyronine (reverse T3): assessment of thyroid gland content and serum measurements in conditions of normal and altered thyroidal economy and following administration of thyrotropin releasing hormone (TRH) and thyrotropin (TSH).

The present report describes the development of a radioimmunoassay for 3,3',5'-L-triiodothyronine (reverse T3) which is performed on unextracted serum. Utilizing this radioimmunoassay, 21 normal subjects had a mean (+/-SD) serum reverse T3 level of 60 +/- 12 ng/100 ml, 17 of 19 hyperthyroid patients had elevated serum reverse T3 levels, and 10 of 11 hypothyroid subjects had decreased serum reverse T3 concentrations. Thyroidal secretion of reverse T3 was assessed by measurements in samples obtained from the internal carotid artery and jugular vein of sheep following the administration of thyrotropin releasing hormone (TRH) or bovine thyrotropin (TSH). Reverse T3 levels were increased 45-60 min after TRH administration, but TSH administration produced inconsistent alterations in reverse T3, although 18 of 27 samples obtained after TSH injection were higher than their average respective baseline concentration and the mean peak reverse T3 level was 14% higher than baseline. Following TRH administration to 10 normal human subjects, mean serum reverse T3 levels significantly increased from 53.6 ng/100 ml to 56.3ng/100 ml (P less than .05). The thyroid gland content of reverse T3 in human autopsy material was 6.5 +/- 1.5 microng/g tissue. Both pregnancy and estrogen administration were associated with increases in serum reverse T3 concentrations presumably because of their ability to augment thyroxine binding globulin synthesis.     

Clinical evaluation of routinely applied highly sensitive TSH-RIA

Forty-one routine TSH assays were carried out by applying newly developed, highly sensitive TSH RIA (T. Mori, et al., Folia Endocrinol. Jap., 56: 1231, 1980). B/B0 percent of the least standard point (0.156 microU/ml) was consistently lower than B0 and higher than that of 0.31 microU/ml. The distributions of assayed TSH concentrations in 1394 sera revealed that 34.8% of the total were in an undetectable range when measured by conventional method (less than 1.0 microU/ml), but this method picked up 16.4% (0.156-1.0 microU/ml), and only 5.4% exceeded the upper limit (20 microU/ml). TRH test results (500 micrograms i.v. bolus) in 45 cases of thyroid disorders with low or normal basal TSH revealed that the peak TSH of those with basal TSH of 0.156-1.0 microU/ml (8.98 +/- 4.15 microU/ml) was significantly different from those of less than 0.156 microU/ml or 1.0-3.2 microU/ml. Further, TSH concentrations in 19 patients after T3 administration (75 micrograms X 7 days) were 0.183 +/- 0.073 microU/ml, and all but one (0.43 microU/ml) showed values lower than the normal range (0.31-3.2 microU/ml). Thyroid hormone concentrations in cases with TSH of 0.156-1.0 microU/ml were limited in the ranges of less than 300 ng/dl T3 and/or less than 15 micrograms/dl T4, and these were considered to be the threshold of definite TSH inhibition.(ABSTRACT TRUNCATED AT 250 WORDS)     

Increased serum thyroglobulin concentrations and impaired thyrotropin response to thyrotropin-releasing hormone in euthyroid subjects with endemic goiter in Sicily: their relation to goiter size and nodularity

Serum thyroglobulin (Tg), T4, T3, FT4, FT3, TSH concentrations and TSH response to iv TRH (delta TSH) were measured in 56 consecutive patients with (multi) nodular goiter from a severely iodine-deficient endemic goiter area in Northeastern Sicily and in 11 non goitrous euthyroid individuals living in the same area. Serum Tg concentrations were sharply increased in goitrous subjects (453 +/- 476 ng/ml) and related to thyroid size and the presence of nodules (chi 2 = 43.5, p less than 0.0005). Serum TSH levels measured in goitrous patients (2.1 +/- 0.9 microU/ml) were significantly lower than those measured in nongoitrous iodine deficient subjects (3.1 +/- 0.9 microU/ml, p less than 0.001) and decreased with increasing goiter size and nodularity (chi 2 = 27.3, p less than 0.05). A similar pattern was shown by the analysis of the delta TSH (chi 2 = 43.1, p less than 0.0005). These results suggest that at least a part of the largest and multinodular goiters become autonomously functioning with duration and growing in size. In 13 goitrous patients with absent or impaired response to TRH, a significant direct relation was apparent between log-Tg and goiter size and nodularity (r = 0.64) with an inverse relationship between serum FT3 and delta TSH (r = 0.73). A computed program analysis based on the combination of different independent variables (x) including age, thyroid size and nodularity, serum TSH, log-Tg and FT3, indicated the existence of a significant negative relationship between these variables and the TSH response to TRH (r = 0.75, p = 0).(ABSTRACT TRUNCATED AT 250 WORDS)