Dominant inhibition of thyroid hormone action selectively in the pituitary of thyroid hormone receptor-beta null mice abolishes the regulation of thyrotropin by thyroid hormone

Thyroid hormones, T4 and T3, regulate their own production by feedback inhibition of TSH and TRH synthesis in the pituitary and hypothalamus when T3 binds to thyroid hormone receptors (TRs) that interact with the promoters of the genes for the TSH subunit and TRH. All TR isoforms are believed to be involved in the regulation of this endocrine axis, as evidenced by the massive dysregulation of TSH production in mice lacking all TR isoforms. However, the relative contributions of TR isoforms in the pituitary vs. the hypothalamus remain to be completely elucidated. Thus, to determine the relative contribution of pituitary expression of TR-alpha in the regulation of the hypothalamic-pituitary-thyroid axis, we selectively impaired TR-alpha function in TR-beta null mice (TR-beta-/-) by pituitary restricted expression of a dominant negative TR-beta transgene harboring a delta337T mutation. These animals exhibited 10-fold and 32-fold increase in T4 and TSH concentrations, respectively. Moreover, the negative regulation of TSH by exogenous T3 was completely absent and a paradoxical increase in TSH concentrations and TSH-beta mRNA was observed. In contrast, prepro-TRH expression levels in T3-treated TR-beta-/- were similar to levels observed in the delta337/TR-beta-/- mice, and ligand-independent activation of TSH in hypothyroid mice was equivalently impaired. Thus, isolated TR-beta deficiency in TRH paraventricular hypothalamic nucleus neurons and impaired function of all TRs in the pituitary recapitulate the baseline hormonal disturbances that characterize mice with complete absence of all TRs.     

Release of thyroid stimulating hormone from chick anterior pituitary glands by thyrotropin releasing hormone (TRH)

Chicks two and ten days-of-age respond to a wide range of thyrotropin releasing hormone (TRH) dosages as measured by thyroid uptake of ³²P. The duration of hormone and ³²P action is important. Excellent responses were obtained with the injection of 1.0 μCi³²P at one hour and TRH either at one or four hours before autopsy in both two-day and ten-day-old birds. The ³²P uptake in the thyroid glands was increased by doses of hormone which ranged from 40 nanograms to 125,000 nanograms and was bimodal. Analysis of the data when calculated using log¹⁰ of dose was best accomplished by the use of 5th-degree polynomial equations. It is suggested that the bimodal response is a result of a dual action of TRH. First, TRH initiates the release of stored TSH from the anterior pituitary; and second, TRH stimulates the secretion of newly synthesized TSH by the anterior pituitary.     

Serum thyrotropin response to thyrotropin-releasing hormone and free thyroid hormone indices in patients with familial thyroxine-binding globulin deficiency.

The response in serum thyrotropin (TSH) to synthetic thyrotropin-releasing hormone (TRH) as well as serum free thyroxine index (FT4I) and free triiodothyronine index (FT3I) was investigated in six patients with familial thyroxine-binding-globulin (TBG) deficiency. The total serum thyroxine (T4) and triiodothyronine (T3) concentrations were significantly decreased, compared with those of normal subjects (3.4 +/- 0.9 microgram/dl, mean +/- SD. vs. 9.0 +/- 1.5 microgram/dl, p less than 0.01 and 87 +/- 27 ng/dl vs. 153 +/- 37 ng/dl, p less than 0.01, respectively). FT4I was lower than the normal range in all but one (5.3 +/- 1.5 vs. 8.9 +/- 1.6, p less than 0.01), whereas FT3I was all in the normal range and of no significant difference from the normal control (132 +/- 22 vs. 148 +/- 25). Serum TSH concentrations in TBG deficiency were all in the normal range (1.0-4.2 muU/ml) and the maximum TSH increments following TRH 500 microgram iv were 8.9 +/- 2.0 muU/ml and of no significant difference from the normal control (10.2 +/- 4.5 muU/ml). These results indicate that the euthyroid state in familial TBG deficiency is more clearly defined by TRH-test and the normal response to TRH in familial TBG deficiency is presumably under the control of the serum free T3 level rather than the serum free T4 level.     

Effects of thyrotropin and thyroid hormones in vivo on thyroid responsiveness to thyrotropin in vitro.

The thyroid gland of rats fed propylthiouracil is known to be unresponsive in vitro to thyrotropin; to investigate further the underlying mechanism groups of rats were variously treated with propylthiouracil and thyroid hormone or subjected to hypophysectomy. In vitro responsiveness of the thyroids was tested by measuring an increase in the concentration of c AMP when thyrotropin or prostaglandin E1 was added to the medium. Results showed that responsiveness to thyrotropin partially returned with rats fed prophylthiouracil and hypophysectomized 5, but not 2, days before death; hypophysectomy of normal rats led to increased in vitro responsiveness to thyrotropin and this was partially reversed by injections of thyrotropin for a week before death. Administration of thyroid hormone had little effect in these investigations and in vitro responsiveness to prostaglanding E1 was not consistently influenced by any of the in vivo regimens. From this experience we conclude that, at least as studied in vitro, circulating thyrotropin has a significant role in modulating responsiveness of the thyroid to thyrotropin.     

Regulation of thyrotropin biosynthesis. Discordant effect of thyroid hormone on alpha and beta subunit mRNA levels

We have studied the regulation of the biosynthesis of thyrotropin (TSH) and its alpha and beta subunits by thyroid hormone in thyrotropic tumors carried in hypothyroid mice. Treatment with 3,5,3'-triiodo-L-thyronine (T3) (20 micrograms/100 g, body weight) daily for 4 or 10 days reduced serum TSH to 3 and 0.3% of control, respectively. Serum levels of free alpha subunit were reduced to 60 and 11% of control at 4 days and 10 days, respectively, and serum free TSH-beta was undetectable at both time points. There was no significant decrease in tumor TSH content after 4 days of treatment and, after 10 days, TSH content was reduced to 15% of control levels. There was no significant effect of T3 on tumor alpha subunit levels at either 4 or 10 days. In contrast, tumor TSH-beta content was markedly reduced after 4 days and 10 days of T3 treatment, to 29 and 10% of control levels, respectively. Translation of tumor poly(A) mRNA in a rabbit reticulocyte lysate system showed that thyroid hormone decreased translatable TSH-beta mRNA to undetectable levels at both 4 and 10 days, whereas translatable alpha mRNA was reduced strikingly only at 10 days in one of two tumors. RNA blot hybridization with 32P-labeled plasmid probes containing alpha or TSH-beta cDNAs showed that TSH-beta mRNA was reduced to less than 10% of control after both 4 and 10 days of T3 treatment, whereas, again, alpha mRNA was only reduced in one of two tumors at 10 days. Our data thus show that thyroid hormone affects alpha and TSH-beta mRNA and protein levels discordantly and suggest that regulation of TSH biosynthesis may occur predominantly at the level of TSH-beta mRNA.     

Growth hormone response to thyrotropin releasing hormone in a pellagrin

An abnormal hyperresponse of GH to intravenous injection of TRH in a 66-year-old female pellagra patient with typical 3'D's was reported. Diagnosis of pellagra was mainly based on her clinical course and manifestations, although serum levels of nicotinic acid and serotonin were within the normal range. Serum vitamin A and B2 levels were low. However, these findings did not exclude the diagnosis. The abnormal GH response to TRH observed in this patient was decreased at 2 months and thoroughly disappeared at 10 months after admission. GH response to arginine showed an exaggerated and sustained response on admission, decreased at 2 months and showed an almost normal pattern at 10 months after admission. TSH and prolactin response to TRH were normal throughout the clinical course. LH and FSH response to LH-RH were exaggerated, suggesting post-menopausal hypogonadism. Cortisol response to ACTH showed slightly sustained reactions at both times of the provocation. Oral glucose tolerance test revealed a slight impairment in this patient. These results suggest that pellagra is one of the disorders which exhibit an abnormal hyperresponse of GH to intravenous administration of TRH.     

Increased thyrotropin binding in hyperfunctioning thyroid nodules

The object of this study was to investigate TSH receptors in hyperfunctioning thyroid nodules (HFN). In HFN, obtained from seven patients, 125-I-TSH binding as determined by equilibrium binding analysis on particulate membrane preparations, was found to be significantly increased as compared with normal thyroid tissues (five patients; P less than 0.001). Scatchard analysis of TSH-binding revealed two kinds of binding sites for both normal thyroid tissue and HFN, and displayed significantly increased association constants of high- and low-affinity binding sites in HFN (Ka = 11.75 +/- 6.8 10(9) M-1, P less than 0.001 and Ka = 2.1 +/- 1.0 10(7) M-1, P less than 0.025; x +/- SEM) as compared with normal thyroid tissue (Ka = 0.25 +/- 0.06 10(9) M-1, Ka = 0.14 +/- 0.03 10(7) M-1; x +/- SEM). The capacity of the high-affinity binding sites in HFN was found to be decreased (1.8 +/- 1.1 pmol/mg protein, x +/- SEM) in comparison with normal thyroid tissue (4.26 +/- 1.27 pmol/mg protein; x +/- SEM). TSH-receptor autoradiography applied to cryostatic tissue sections confirmed increased TSH binding of the follicular epithelium in HFN. These data suggest that an increased affinity of TSH-receptor sites in HFN in iodine deficient areas may be an important event in thyroid autonomy.     

Plasma growth hormone and thyrotropin responses to thyrotropin releasing hormone in freely behaving and urethane anesthetized rats.

Plasma GH and TSH responses to thyrotropin releasing hormone (TRH) were examined in freely behaving and urethane anesthetized rats. The i.v. administration of TRH (200ng/100g b.wt.) resulted in consistent elevations of plasma GH only in urethane anesthetized rats, while significant elevations of plasma TSH were similarly observed in both conditions. Results suggest that urethane influences plasma GH responses to TRH.