Stimulation with thyrotrophin-releasing hormone (TRH) during antithyroid treatment.

During antithyroid treatment a total of 88 TRH tests was performed in 56 clinical euthyroid patients. 56% had negative response to TRH (i.e. delta TSH smaller than 2 muU/ml) after being treated in average 12.6 months and no relation between the duration of treatment and the outcome of the TRH test was found. In the group with positive TRH tests (i.e. delta TSH greater than 2 muU/ml) the mean T4 value was slightly decreased (5.8 plus or minus SD 2.5 mug/100 ml) while the mean T3 value was normal (121 plus or minus SD 32 ng/100 ml). The group with negative TRH tests had quite normal serum T4 values (9.3 plus or minus SD 3.1 mug/100 ml) but in general high normal or elevated serum T3 values (175 plus or minus SD 31 ng/100 ml). Our results seem to indicate that serum T3 is of greater importance than serum T4 with regard to the outcome of the TRH test. The majority of the cases with negative TRH tests, however, had serum T3 and T4 values within normal range. In almost all patients with a negative TRH test a negative T3 suppression of 131I uptake in the thyroid gland was found while a positive TRH test was not correlated with suppressibility of 131I uptake.     

Pharmacokinetics of thyrotrophin-releasing hormone in patients in different thyroid states

In view of recent reports suggesting that thyroid hormone control of TRH degradation occurs outside the central nervous system in animals, the effect of thyroid status on serum and tissue degradation of TRH in man was investigated. In six patients with hyperthyroidism and six patients with hypothyroidism, constant TRH infusions were carried out for determination of plasma clearance rate (PCR) and half-life of disappearance (t1/2) of TRH, with simultaneous determination of half-life of disappearance in serum in vitro (t1/2p). Using a kinetic model, this enabled the calculation of the half-life of disappearance in the extravascular tissue compartment (t1/2t). All patients were reinvestigated after they had become euthyroid. PCR, t1/2 and t1/2p were 22.1 +/- 3.4 ml/kg per min, 6.8 +/- 1.1 min and 17.3 +/- 6.7 min (means +/- S.D.) respectively in the euthyroid patients. The t1/2p was slightly but significantly prolonged during hyperthyroidism. The t1/2 was 5.6 min in the hyperthyroid patients compared with 9.4 min in the hypothyroid patients. The calculated t1/2t was 6.5 min in the euthyroid patients. In the patients with untreated hyperthyroidism, t1/2t was significantly reduced (22.7 +/- 10.7%; mean +/- S.D.), while it was considerably prolonged (41.1 +/- 24.6%) in patients with untreated hypothyroidism. The percentage reduction or prolongation of t1/2t was negatively correlated with the logarithm of the serum concentrations of thyroxine (r = 0.92) and tri-iodothyronine (r = 0.91) in the untreated patients. Thus, thyroid hormones induce alterations in the pharmacokinetics of TRH. This may partly be due to induction by thyroid hormones of membrane-bound pyroglutamyl aminopeptidase.     

The thyrotrophin-releasing hormone (TRH)-like peptides in rat prostate are not formed by expression of the TRH gene but are suppressed by thyroid hormone.

Thyrotrophin-releasing hormone (TRH)-immunoreactive peptides were extracted from rat prostate and divided into two groups by mini-column cation exchange chromatography. The amounts of the peptides in each group were determined by radioimmunoassay with a TRH antiserum. The unretained peptides which lacked a basic group and the retained peptides which possessed a basic group were further purified by high-performance liquid chromatography. The unretained fraction was found to contain a series of TRH-immunoreactive peptides, one of which corresponded chromatographically to synthetic pGlu-Glu-Pro amide and another to pGlu-Phe-Pro amide. None of the TRH-immunoreactive peptides in either fraction exhibited the chromatographic behaviour of TRH. Additional evidence for the absence of TRH gene expression in the prostate was obtained by Northern blot analysis and by application of polymerase chain reaction amplification, which failed to reveal TRH mRNA. Furthermore the preproTRH-derived peptide, preproTRH(53-74), could not be detected by radioimmunoassay. The influence of thyroid status was investigated on the levels of the TRH-like peptides in the prostate. Adult rats were treated chronically with thyroxine (T4) or propylthiouracil (PTU) and the concentrations of the TRH-immunoreactive peptides were determined by chromatography and radioimmunoassay. Treatment with T4 caused the levels of the neutral and acidic TRH-like peptides to fall to approximately one-third of the levels in the controls. No significant difference from the controls was seen in the concentrations of the peptides in the prostates of rats rendered hypothyroid by administration of PTU. The results demonstrate that rat prostate contains TRH-immunoreactive peptides which are not derived from the TRH gene.(ABSTRACT TRUNCATED AT 250 WORDS)     

Desensitization of thyrotrophin-releasing hormone (TRH)-induced growth hormone secretion in chickens: coincident down-regulation of TRH binding to pituitary membranes

Release of GH is stimulated by TRH in chickens. However, for 60 min following a priming injection of TRH, a second injection of TRH is unable to provoke further GH release. TRH binds to the plasma membranes of the pituitary caudal lobe, in which somatotrophs predominate, although the magnitude of [3H]3-methyl-histidine2-TRH ([3H]Me-TRH) binding was reduced (by 25-50%) 30 min after the i.v. administration of a dose of TRH (5 micrograms/kg) maximally effective in provoking GH release. A significant reduction in [3H]Me-TRH binding to caudal lobe membranes was also observed within 15 min of TRH administration and was maintained for at least 60 min. Control levels of [3H]Me-TRH binding were restored 2 h after TRH injection, coincident with the restoration of GH responsiveness to TRH challenge. The suppression of [3H]Me-TRH binding to pituitary membranes 30 min after in-vivo TRH administration was dose related, whereas the maximal (10 min) GH response to TRH was biphasic. The suppression of [3H]Me-TRH binding to chicken pituitary membranes was due to direct pituitary actions of TRH and could be induced by a 30-min exposure to 100 nM TRH in vitro. These results demonstrate that avian pituitary TRH-binding sites differ greatly from mammalian ones in the timing of the onset and duration of down-regulation. Pituitary TRH-binding sites in birds are rapidly and transiently down-regulated following TRH administration in vivo, coincident with the period of GH refractoriness to TRH challenge.     

Thyroid hormone modulation of thyrotrophin-releasing hormone (TRH) and TRH-Gly levels in the male rat reproductive system

Thyrotrophin-releasing hormone (TRH) occurs in high concentrations in the rat ventral prostate and its concentrations is regulated in a positive dose-response manner by testosterone in castrated rats. alpha-Amidation of the tetrapeptide precursor, TRH-Gly, is a rate-limiting step in TRH biosynthesis. To investigate further the hormonal regulation of TRH biosynthesis in prostatic tissue, Sprague-Dawley rats of approximately 250 g were injected s.c. with either physiological saline or 3 mg propylthiouracil (PTU) daily for 5 days. The reproductive tissues were boiled in acetic acid (l mol/l), dried and extracted with methanol. The methanol extracts were measured for TRH immunoreactivity (TRH-IR) and TRH-Gly-IR by radioimmunoassay. Hypothyroidism induced by PTU significantly increased TRH-IR and TRH-Gly-IR levels in prostate and testis and reduced these levels in epididymis but did not affect the serum concentrations of testosterone compared with those of controls. Corresponding changes in TRH and TRH-Gly in the rat prostate were established by high-pressure liquid chromatography. To control for possible pharmacological effects of PTU on TRH biosynthesis, additional experiments were carried out on castrated rats receiving testosterone replacement and treatment with PTU plus methimazole. Treatment with thyroxine (T4) significantly reduced the increase in prostatic TRH levels due to hypothyroidism, despite the drug-induced blockade of the conversion of T4 to tri-iodothyronine. These effects parallel similar observations made in rat spinal cord and pancreas. This study demonstrates that in the male rat reproductive system the levels of TRH and its immediate biosynthetic precursor, TRH-Gly, are regulated by thyroid hormones.     

Stimulation of growth hormone secretion in dwarf chickens by thyrotrophin-releasing hormone (TRH) or human pancreatic growth-hormone-releasing factor (hpGRF)

The basal plasma growth hormone (GH) level in adult sex-linked dwarf hens was elevated in comparison with autosomal dwarf hens and with control (Cornell K strain) laying hens. The iv administration of thyrotrophin-releasing hormone (TRH) (10 μg/kg) had no effect on GH secretion in control hens but slightly (1.2-fold) and transiently (for 10 min) increased the GH level in the autosomal dwarfs and greatly (8.7-fold) increased the GH level in the sex-linked dwarfs, in which it remained elevated for at least 30 min after injection. The iv administration of human pancreatic GH-releasing factor (hpGRF) (10 μg/kg) stimulated GH release in each strain. The response in the sex-linked dwarfs was greater than that in the autosomal dwarfs and the control hens but less than that elicited by TRH. These results suggest that the increased basal GH level in the sex-linked dwarfs results from an increased responsiveness to provocative stimulation.     

Comparative stimulation of growth hormone secretion in anaesthetized chickens by human pancreatic growth hormone-releasing factor (hpGRF) and thyrotrophin-releasing hormone (TRH)

Plasma concentrations of growth hormone (GH) were elevated in anaesthetized male domestic fowl following the intravenous administration of either synthetic human pancreatic GH-releasing factor 1-44 (NH2) (hpGRF) or synthetic thyrotrophin-releasing hormone (TRH). In 6-week-old chicks the plasma GH level was elevated between 5 and 10 min after the injection of hpGRF at doses between 1 and 80 micrograms/kg. The magnitude of the response increased with doses of hpGRF between 1 and 10 micrograms/kg but declined with higher doses. The GH concentration rapidly declined between 10 and 20 min and between 20 and 40 min after injection. The administration of TRH had similar effects on GH secretion, although the responses were greater than with comparable doses of hpGRF, and the most effective dose (1-1.4 micrograms/kg) was less than with hpGRF. In anaesthetized adult cockerels GH secretion was also increased by the administration of hpGRF (1-20 micrograms/kg) or TRH (0.1-80 micrograms/kg) and in both cases the dose-response relationship was biphasic. The maximal response to TRH in adult birds was again greater than that produced by hpGRF although the response was less than that elicited in immature birds and required a higher dose (20 micrograms/kg) of TRH. The optimal dose of hpGRF and the magnitude of the GH response induced in adult birds was comparable with that in immature chicks. These results demonstrate provocative effects of TRH and hpGRF on GH secretion in the domestic fowl. The sensitivity of the GH response to TRH suggests that it may have a physiological role in the hypothalamic control of GH secretion.     

Plasma immunoreactive thyrotropin releasing hormone (TRH) values in normal newborns.

The study reported here extends investigation on the pituitary thyroid axis in newborn infants, including the assay of plasma immunoreactive TSH levels at different intervals after delivery. Blood samples were collected at birth and after 30, 60, 120 minutes, 6, 24 and 48 hours. Plasma TRH levels were also estimated in normal adult subjects and pregnant women. No significant difference was observed with regard to sex, pregnancy or age, except for a marked increase in newborn infants after delivery. Plasma TRH values, already moderately high at birth (mean 46 pg/ml, range 34-57) reached rapidly a peak of 78 pg/ml (range 60-93) 30 minutes after delivery, decreased rapidly between 30 minutes and 2 hours post-partum, then fell gradually to normal range at 24 hours. A comparison of plasma TRH and TSH levels measured simultaneously suggests that the acute TSH surge at delivery is mediated by TRH secretion.     

Thyrotropin releasing hormone (TRH) immunoreactivity and thyroid function in obesity

Circulating TRH-immunoreactive levels, the thyrotropin response to a TRH intravenous stimulation (200 micrograms) and thyroid hormone concentrations have been determined in 43 overweight subjects (body mass index 45 +/- 12 kg/m2, mean +/- s.d.) and 46 (body mass index 22 +/- 2 kg/m2) normal weight controls. The TRH levels measured by a recently developed, highly specific radioimmunoassay were similar among both groups (44 +/- 16 vs 40 +/- 12 fmol/ml, n.s.). The pattern of response of TSH to TRH was normal in the obese and no significant difference was observed between the peak TSH values of the obese and the normal group (8.3 +/- 2.8 vs 8.7 +/- 2.2 microU/ml, n.s.). No correlations were found between the degree of obesity and the concentrations of TRH, TSH and peripheral thyroid hormone levels. Three obese patients showed a delta-TSH of 18, 19 and 21 microU/ml at normal thyroid hormone concentrations as sign of latent hypothyroidism. These data indicate that in obesity: (a) the TSH response to i.v. TRH is not impaired, (b) circulating TRH-IR levels are not significantly changed and (c) the incidence of overt hypothyroidism is not increased.     

Fasting-induced changes in thyroid hormone economy in normal and acromegalic subjects.

To study whether high growth hormone (GH) milieu may counteract fasting-induced changes in thyroid hormone economy, we measured basal and TRH-stimulated TSH concentrations as well as thyroxine (total, TT4 and free, FT4), total triiodothyronine (TT3) and total reverse T3 (TrT3) before and after a 6-day fast in 6 healthy men and in 8 patients with acromegaly. Baseline values for all parameters were similar in both groups. Fasting induced similar increases in TT4 and TrT3 concentrations and a similar decline in TT3 concentrations in both groups. The TT3/TT4 and TrT3/TT4 ratios changed identically in both groups. We conclude that high GH is incapable of altering fasting-induced changes in thyroid hormone economy.     

Intra- and extravascular turnover of thyrotrophin-releasing hormone in normal man

In 14 normal subjects constant TRH infusions for determination of plasma clearance rate (PCR) and half-life of disappearance (t 1/2) of TRH were carried out with simultaneous determination of half-life of disappearance of TRH in serum in vitro (t 1/2p). PCR, t 1/2 and t 1/2p were 1532 +/- 423 ml/min, 6.6 +/- 1.5 min and 16.8 +/- 9.4 min respectively (mean +/- S.D.) and displayed only minor fluctuations when determined repeatedly in the same subjects (coefficients of variation within individuals were 15.1, 10.6 and 7.5% respectively). Simultaneous determination of PCR, t 1/2 and t 1/2p enabled calculation of the half-life of disappearance of TRH in the extravascular tissue compartment (t 1/2t). Values of t 1/2t (6.3 +/- 1.4 min) correlated to t 1/2p (r = 0.95). Activities of TRH-degrading enzymes in tissues and in serum were independent of sex, phase of female menstrual cycle, time of day and of the concentrations of TRH used. The methods employed for this investigation offer the possibility of examining the degradation of TRH and TRH analogues both in the serum and in the extravascular compartment during various conditions.     

Biochemistry of thyroid regulation under normal and abnormal conditions

Perhaps in an oversimplified view, abnormal thyroid growth can be classified into two main categories: a) those cases due to excess of thyroid stimulators extrinsic to the gland; b) situations in which an intrinsic alteration in the gland occurs: Extrinsic (excess thyroid stimulation) Iodide deficiency with elevated TSH Goitrogens Graves' immunoglobulins Thyroid stimulating factors produced by tumors Dishormonogenesis with hypothyroidism Intrinsic (normal TSH) Increased sensitivity to TSH (iodine depletion) Altered autoregulation (?) Abnormal TSH receptor Other biochemical abnormalities From the studies performed in animals it can be concluded that since goiter appears before a detectable increase in serum TSH occurs, an intrinsic alteration in the thyroid gland would be responsible for the onset of growth. Under these conditions TSH would play a permissive role in promoting and maintaining the gland enlargement. In some aspects this situation is similar to that of certain endemic goiter areas. It may be postulated that under a mild iodine deficiency a decrease in thyroidal iodine concentration occurs (and/or in certain iodocompounds), thus rendering the gland more sensitive to the stimulatory action of TSH, and leading to the appearance of goiter. If this mechanism is able to maintain an euthyroid status no further alterations will occur. In more severely iodine deficient areas, or when additional factors such as dietary goitrogens are present, hypothyroidism develops and TSH is clearly elevated. A similar localized mechanism can be postulated for the development of nodular goiter. It is more difficult to explain the pathogenesis of goiter and tumors in nonendemic areas, since the biochemical findings so far reported are not conclusive. It seems likely that an alteration of the TSH receptor is a common factor to many tumors in man and animals. However, some contradictory results would preclude us from making a general statement. The wide variety of biochemical alterations reported would perhaps indicate, that there is not a single cause for the rise of abnormal thyroid growth and that different factors may play a role in the regulation of growth under such circumstances. It is to be hoped that future studies will provide a better comprehension of this problem.