Pituitary Thyrotropin Research Articles

Human pituitary thyrotropin: Primary structure of the hormone specific β subunit

The primary structure of the hormone specific β subunit of human pituitary thyrotropin has been deduced from the composition and complete or partial amino acid sequences of the tryptic and chymotryptic peptides. It consists of 112 amino acid residues with the single oligosaccharide moiety which is assumed to be linked to the asparagine residue at position 23.

Serum triiodothyronine, total thyroxine, and thyroxine to triiodothyronine ratios in paired maternal-cord sera and at one week and one month of age.

Extract: Twenty-seven paired maternal-cord sera were analyzed for serum triiodothyronine (T3), total serum thyroxine (TT4), and maximum thyroxine globulin binding capacity (TBG cap.). In 17 of the infants the same studies were repeated at 1 week of age and in 10 of the 17 at 1 month. The mean T3 value for the cord sera was 73.1 ± 20.4 ng/100 ml while the mean value for the maternal sera was 204 ± 49.2. The difference in the mean values was highly significant (P < 0.001). The maternal T3 values are comparable with those of nonpregnant control subjects; those of cord blood fall below normal values. No statistically significant difference was found between the TT4 and TBG cap. values for cord and maternal sera. Both values were higher than the normal nonpregnant control values. At 1 week of age the serum T3 levels had increased to a mean of 208.4 ± 40.9 ng/100 ml. This mean is significantly different from the one observed in cord sera (P < 0.001). A significant increase in the TT4 values, mean 12.53 ± 2.2 μg/100 ml (P < 0.01), was observed by the end of the 1st week. TBG cap. did not change significantly between birth and 1 week of age; by the end of the 1st month of life the serum TT4 had decreased significantly to a mean of 9.5 ± 2.7 μg/100 ml (P < 0.05), while the serum T3 was maintained at a mean level of 228.2 ± 16.9 ng/100 ml, a value not significantly different from the one observed at 1 week of age. Speculation: The extremely low levels of T3 in the cord blood in the presence of TT4 and TBG cap. values equal to those of the maternal blood suggest minimal materno-fetal placental transfer of T3. We have postulated that the low fetal T3 is responsible for the high levels of pituitary thyrotropic hormone (TSH) present in cord blood and early in the postnatal period and that the high levels of TT4 with normal T3 levels observed by the end of the 1st week of life indicate defective peripheral deiodination of thyroxine (T4) to T3 and account for the lack of signs and symptoms of hyperthyroidism. By the end of 1 month of age TT4 and T3 values are within normal ranges, which indicates maturation in the metabolic pathway of T4 and T3.

The Subunits of Human Pituitary Thyroid-stimulating Hormone: ISOLATION, PROPERTIES, AND COMPOSITION

Abstract The subunits, α and β, of human thyroid-stimulating hormone (TSH) from the pituitary have been obtained by the same procedures utilized for the preparation of bovine TSH subunits, namely, dissociation in 1 m propionic acid and separation by gel filtration. The amino acid composition of human TSH-α indicates that this subunit is identical or nearly so with the α subunits of human luteinizing hormone and human chorionic gonadotropin. The NH2-terminal residue is valine and it appears that human TSH-α is shortened at its amino end by 7 residues relative to bovine TSH-α; a similar relationship has been shown by others for human LH-α relative to the ovine or bovine subunit. The β subunit of human TSH differs markedly in amino acid composition from those of the two human gonadotropins, particularly with respect to tyrosine, isoleucine, valine, proline, arginine, and lysine. The carbohydrate analyses for the 2 subunits are given. A major difference between human and bovine TSH is the presence of sialic acid in the former, nearly all in the α subunit. Immunological comparisons by double diffusion show that antisera to the intact hormone or its α subunit cross-react with homologous TSH and with the human gonadotropins but that the antiserum against the β subunit is specific for thyroid-stimulating hormone. The human subunits were shown, by gel electrophoresis and bioassay, to form intra- and interspecies recombinants with complementary subunits from other glycoprotein hormones.

Assessment of thyrotropin-releasing hormone and thyrotropin reserve in man.

Endogenous thyrotropin-releasing hormone (TRH) reserve and pituitary thyrotropin (TSH) reserve were assessed in four normal subjects, three patients post-cryohypophysectomy, one patient with a hypothalamic lesion secondary to trauma, and four patients with Sheehan's syndrome. TSH reserve was determined by the immunoassayable TSH response to 500 mug TRH given i.v. (TRH stimulation test). TRH reserve was assessed by the rebound response in thyroidal iodine release (TIR) following withdrawal of pharmacologic doses of prednisolone (glucocorticoid withdrawal test). When compared with normals, the post-cryohypophysectomy patients demonstrated parallel impairment of TRH stimulation and glucocorticoid withdrawal testing. The patient with the hypothalamic lesion and the four patients with Sheehan's syndrome all had normal TRH stimulation tests, indicating adequate TSH reserve capacity, yet had abnormal glucocorticoid withdrawal tests, indicative of impairment in endogenous TRH reserve or neurohumoral transport. Three of the patients (hypothalamic injury and two Sheehan's) with impaired TRH reserve were euthyroid. THE FOLLOWING CONCLUSIONS WERE REACHED: (a) A combination of the TRH stimulation test and glucocorticoid withdrawal test may allow for differentiation between pituitary and suprahypophyseal disorders. (b) Certain cases of Sheehan's syndrome appear to have impaired endogenous TRH reserve or transport. (c) Euthyroidism can be maintained in spite of diminished TRH reserve.

Effect of thyrotropin-releasing hormone on human pituitary thyrotropin, prolactin, placental lactogen, and chorionic thyrotropin.

The effect of synthetic thyrotropin releasing hormone (TRH) on serum thyrotropin (hTSH), prolactin (hPRL), chorionic thyrotropin (hCT), and placental lactogen (hPL) was investigated in 13 normal pregnant women who had therapeutic abortions at 8–22 weeks of gestation. Five hundred μg TRH injected iv increased serum hTSH consistently in all subjects. The baseline serum hTSH was 6.1 ± 0.7 (SE) μU/ml, and the mean maximum increment was 26.3 ± 4.7 μU/ml. Serum hPRL levels also increased in all subjects after giving TRH. The baseline serum hPRL was 27.5± 4.5 ng/ml, and the mean maximum increment was 37.0 ± 7.4 ng/ml. The minimum level of detection of hCT was 2.5 μU/ml based on its cross-reaction in a radioimmunoassay for bovine pituitary TSH. Serum hCT was detected in sera of only 4 pregnant women and showed no significant change after TRH. Serum hPL levels varied from 1.1 ng/ml to 2.8 μg/ml and showed no significant change after TRH. Studies were also carried out to determine the effect of TRH in vitro on fragments of human full-term placentas incubated in Krebs-Ringer bicarbonate. TRH did not alter the concentrations of hPL in the incubation medium nor was there any effect of TRH on the thyrotropic activity of the tissue measured by radioimmunoassay and bioassay. These results indicate that TRH stimulates release of both hTSH and hPRL in pregnancy and does not affect the release of hPL or hCT. The control of the secretion of these placental hormones remains unknown.

Observations on the thyroid gland in rats following the administration of sulfamethoxazole and trimethoprim

Rats receiving sulfamethoxazole exhibited hyperplasia which progressed to nodularity or adenoma formation after 52 and 60 weeks. Local vascular invasion and lung metastases were observed in some rats sacrificed after 60 weeks' treatment. Thyroid hyperplasia was not observed in monkeys in a 52-week study with sulfamethoxazole. The concomitant administration of trimethoprim was without effect on the thyroid of either species. In another rat study, withdrawal of sulfamethoxazole from the diet after 6 or 13 weeks' administration resulted in involution of the thyroid follicular epithelium so that the follicular structure resembled that of untreated controls. The goitrogenic action of sulfonamides in the rat is not believed to be a direct effect on the thyroid but an indirect effect mediated by the excessive release of pituitary thyrotropin in response to a deficiency of circulating thyroid hormone. This hormonal imbalance is not readily produced by sulfonamides in primates.

Human pituitary thyrotropin: Isolation and chemical characterization of its subunits

The subunits of human pituitary thyrotropin have been separated and purified by countercurrent distribution and exclusion chromatography. The NH 2-terminal sequence of the β subunit is identical to that of the β subunit of bovine thyrotropin. However, amino acid composition and peptide map of tryptic and chymotryptic digests as well as compositions of tryptic and chymotryptic peptides suggest that the amino acid sequence of the α subunit is identical to that of the α subunit of human interstitial cell stimulating hormone.

Thyrotropin releasing hormone: secretion and actions.

Extensive physiologic and anatomic studies have demonstrated the central role of the anterior hypothalamus in the neurohumoral control of pituitary thyrotropin (TSH) secretion and biosynthesis (1, 2). The structural elucidation in 1969 of thyro­ tropin releasing hormone (TRH), isolated from more than 100,000 hypothalamic fragments derived from pigs (3) and sheep (4), constituted a momentous achieve­ ment in endocrine research. The subsequent large scale synthesis and availability of TRH has catalyzed vigorous activity in multiple areas of basic and clinical investiga­ tion. The progress made in understanding the structure, synthesis, secretion, de­ gradation, and actions ofTRH forms the substance of the present chapter. Excellent reviews on various aspects ofTRH physiology have been published recently (5-9).

Fractionation and assay of chicken pituitary hormones

Synopsis The glycoprotein hormones of the chicken pituitary gland, follicle‐stimulating hormone (FSH), luteinising hormone (LH) and thyroid stimulating hormone (TSH), have been fractionated and partially purified. Fractions were assayed for gonadotrophins using the testicular uptake of 32P in 1‐d‐old chicks and for thyro‐trophin by chick thyroidal 32P uptake.

Adenosine 3′,5′-cyclic phosphate phosphodiesterase from bovine thyroid: Isolation and properties of a partially purified, soluble fraction

1. 1.|Adenosine 3′,5′-cyclic phosphate (cyclic AMP) phosphodiesterase was partially purified from a 10 000 × g supernate of bovine thyroid; the enzyme preparation so obtained was free of 5′-nucleotidase and deaminase activity and yielded [ 3H]-adenosine 5′-phosphate (AMP) as the end product of enzymatic hydrolysis of cyclic [ 3H]AMP. 2. 2.|Using an assay procedure that relied on physiologic substrate concentrations ( i.e confirming to known intracellular levels of cyclic AMP in thyroid), the partially purified thyroid phosphodiesterase was kinetically characterized and compared with a highly purified beef heart phosphodiesterase preparation. Double reciprocal plots of cyclic AMP hydrolysis yielded two apparent Michaelis constants: the lower in the 10 −6 M and the higher in the 10 −5 M range. Two apparent K m values were also obtained for Mg 2+ which was required for expression of maximal enzymatic activity. Other bivalent cations (Mn 2+, Co 2+, Ca 2+) activated the enzyme to a significantly lesser extent; Zn 2+ was stimulatory at 10 −4 M and inhibitory at ≥ 10 −3 M. 3. 3.|A number of cyclic nucleotides and acylated derivatives thereof interfered with the enzymatic hydrolysis of cyclic [ 3H]AMP to varying degrees; of these the mono- and dibutyryl derivatives of cyclic AMP were the most potent inhibitors. The inhibition was competitive in nature with K i values for dibutyryl cyclic AMP, N 6-monobutyryl cyclic AMP, and O 2′-monobutyryl cyclic AMP of 1.6·10 −4 M, 1.1·10 −4 M and 2.6·10 −5 M, respectively. 4. 4.|Methylxanthines and quazodine strongly inhibited thyroid phosphodiesterase whereas sulfonylureas were weakly inhibitory. 5. 5.|High concentrations of pituitary thyrotropin in vitro enhanced thyroid phosphodiesterase activity slightly while heat-inactivated thyrotropin, long-acting thyroid stimulator immunoglobulin G, and the prostaglandins E 2 and F 1 a were ineffective; prostaglandin E 1 was weakly inhibitory. The prostaglandin antagonists 7-oxa-13-prostynoic acid and polyphloretin phosphate effected significant inhibition of thyroid phosphodiesterase, possibly accounting for the agonist properties of these agents occasionally observed in vitro. 6. 6.|Upon complexing with a cyclic AMP-binding protein prepared from beef kidney, cyclic [ 3H]AMP became completely resistant to enzymatic breakdown by thyroidal phosphodiesterase.

The development of lysosomes in the human fetal thyroid in correlation with the onset of functional maturation.

The development of lysosomes in the thyroid epithelium was studied histochemically by light and electron microscopy in 21 human fetuses obtained by hysterotomy for therapeutic abortion between 6 and 20 weeks of gestation. For light microscopy, four acid hydrolytic enzymes, acid phosphatase, diethyl—p—nitrophenyl—phosphateresistant esterase (cathepsin), beta—glucuronidase and beta—glucosaminidase, were used as histochemical lysosomal markers. Acid phosphatase was used as the Lysosomal marker for electron microscopy. Lysosomes of the primary and dense—body type were demonstrated at 6 weeks' gestation and at all stages thereafter. Origin of the lysosomes from the Golgi apparatus was also demonstrated at all stages of development. A rather abrupt increase in the number of lysosomes was noted between the 10th and 12th weeks of gestation, coinciding chronologically with the development of well—defined acinar colloid cavities and, as shown by other authors, with the onset of high—molecular—weight thyroglobulin synthesis, protein iodination by the thyroid epithelium and pituitary thyrotropic hormone secretion. Dilatation of the cisternae of the endoplasmic reticulum, presumably portending increased synthesis of thyroglobulin and other proteins, was also noted at this time. We conclude that, as in the adult thyroid, lysosomal activity is an integral part of the mechanism of thyroid hormone secretion by the fetal thyroid and that its development parallels that of other indices of functional maturity of the fetal thyroid. (Endocrinology91: 438,1972)

Studies of a sibship with apparent hereditary resistance to the intracellular action of thyroid hormone

Further studies are presented on a previously described sibship 1 with a familial syndrome combining deaf mutism, delayed bone maturition, stippled epiphyses, goiter, and high levels of circulating thyroid hormone in the presence of euthyroid clinical state. Seventy-five to 90% of the circulating iodinated compounds were L-thyroxine and L-triiodothyronine by chromatography, ability to bind to thyroxine-binding globulin, L-amino acid oxidase digestion, and precipitation to constant specific activity. The usual basic laboratory tests were normal, and there was no direct evidence of endocrine disease other than that of the thyroid. Although all standard tests of thyroid function, with the exception of the basal metabolic rate (BMR), were typically those of hyperthyroidism, the patients appeared clinically euthyroid. This diagnosis is supported by the normal BMR, serum lipids, cholesterol, tyrosine, enzymes and albumin metabolism, and by normal caloric intake, dentition, and steady growth. Certain tissues appeared to be resistant to thyroid hormone action. We observed delayed bone age, epiphyseal stippling, and low hydroxyproline excretion. Sensorineural deafness, congenital nystagmus, elevated serum carotene, and possibly metachromasia in the cultured skin fibroblasts were also suggestive of hypothyroidism. Endogenous pituitary thyrotropin secretion was not completely suppresed by the high, blood hormone level. Administration of 1 mg/day L-T 4 or 375 μg/day L-T 3 produced some effect on connective tissues. Thyroid gland activity was only partially suppressed with such treatment. Administration of the acetic acid analogue of triiodothyronine had no effect other than thyroid gland suppression, attributed to the liberation of large amounts of iodine. β-Adrenergic blockage and 45 days of propylthiouracil therapy also failed to produce detectable effects. Labeled thyroxine and triiodothyronine studies indicate that these hormones penetrate the liver and other tissues normally or even at an excess rate and are degrated three- to five fold more rapidly than normal. Progressive amelioration of the syndrome has been noted over the past 6–7 yr. The etiology of this syndrome remains unclear. It appears to date to be a congenital. inherited metabolic and possibly transient defect associated with variable degrees of intracellular resistance to the action of thyroid hormone, which has been partially compensated by excess hormone production.

TSH and ACTH secretion after intrapituitary injection of synthetic TRF.

We have compared pituitary thyrotropin (TSH) secretion in response to synthetic thyrotropin—releasing factor (TRF) injected into the anterior pituitary, the median eminence of the hypothalamus, or a peripheral vein. A stereotaxic microinjection procedure was used to inject volumes of one fxl into the pituitary or hypothalamus of pentobarbital anesthetized female rats. The McKenzie bioassay was used to measure plasma TSH. In some experiments the rats were pretreated with dexamethasone to allow simultaneous assessment of adrenocorticotropin (ACTH) secretion, as indicated by fluorometric plasma corticosterone determinations. It was found that intrapituitary injections of 1.0 ng, but not 0.1 or 0.01 ng, were very effective in increasing plasma TSH concentration. In contrast, 1 ng was essentially ineffective when injected into the median eminence or a tail vein. Pretreatment with thyroxine very clearly inhibited the response to one ng of TRF in the pituitary. However, this response was not affected by dexameth...

Synthetic Thyrotropin Releasing Factor as a Test of Pituitary Thyrotropin Reserve

The iv administration of 0.5 mg of synthetic thyroptropin releasing factor (TRF) to control subjects increased plasma TSH from baseline levels of 2.1 ± 0.01 μU/ml to peak values of from 8.5–27 μU/ml. Plasma TSH levels in subjects with primary myxedema were elevated and rose further following TRF. In seven of 16 euthyroid subjects with pituitary tumors, peak values of plasma TSH following TRF ranged from 2.5–6.5 μU/ml indicating limited reserve for TSH secretion. Subjects with hypothyroidism secondary to Sheehan’s syndrome or pituitary tumors did not alter their plasma TSH levels following TRF indicating inadequate pituitary function. Plasma TSH rose normally following TRF in five subjects with TSH deficiency in the absence of a known organic pituitary lesion. This latter group of subjects may have TRF deficiency as a cause of their decreased TSH secretion.

The Pituitary-Thyroid Response to Low Environmental Temperature

This chapter discusses the pituitary–thyroid response to cold: the cold response starts as an open-loop reaction with the following elements: the central nervous system (CNS), pituitary gland, and thyroid gland. After longer periods of cold exposure, thyroid function is controlled by the hormonal negative feedback system between pituitary thyroid stimulating hormone (TSH) secretion and thyroid function, with only a minor modulatory influence of the hypothalamus. The available evidence indicates that this two-phase concept of the thyroidal cold response is highly likely. The timing and the interweaving of the two mechanisms (and phases) of the thyroidal response to cold are still uncertain. On the other hand, a body of data obtained for the greater part after longer periods of cold exposure indicates that the cold response may be due to the following chain of events: increased gastrointestinal loss of triiodothyronine (TH) in the cold, a tendency of the blood level of TH to fall, stimulation of TSH secretion by way of the well-known negative feedback between the secretion of TH and TSH, and increased TH secretion.

Eli Lilly lecture. The subunits of pituitary thyrotropin--their relationship to other glycoprotein hormones.

THIS LECTURE concerns the structure of thyroid stimulating hormone (TSH) and the relationships that have been found between TSH, luteinizing hormone (LH, ICSH) and human chorionic gonadotropin (HCG). Determination of the structures of TSH and LH has required years of purification and chemical studies in many laboratories including our own, but in just the past two years these efforts have resulted in a major increase in our knowledge of the glycoprotein hormones, that is, those polypeptide hormones which contain covalently bound carbohydrate as integral parts of their structures. Particularly important contributions have been made by Condliffe and Bates in the purification of TSH (1) and by Li, Papkoff and their colleagues (2,3) and by Ward and his colleagues (4,5,6) in their structural studies on LH. The major glycoprotein hormones of the pituitary and placenta are listed in Table 1 together with some of their chemical properties as these were known just 2 or 3 years ago. It is obvious that uncertainties...

Response of noraml, hypothyroid and hypothalamo-pituitary insufficient children to synthetic thyrotrophin-releasing hormone.

SUMMARY Thyrotrophin-releasing hormone (TRH) was synthesized by the solid phase technique, administered to 13 children, and the time-course changes in the serum level of thyroid-stimulating hormone (TSH) assessed. In eight normal children, peak levels of TSH occurred 20 min after the injection, and circulating TSH remained significantly raised for 60 min. In three hypothyroid children, the increase in serum TSH was much greater than in normal children, suggesting the existence of large pituitary TSH stores. In two hypopituitary children with TSH deficiency, TSH reserves seemed normal. One of these patients had a craniopharyngioma; after operation, the increase in serum TSH was reduced. These results show that assay of serum TSH after administration of synthetic TRH provides a test which distinguishes pituitary from hypothalamic defects affecting TSH secretion.

Changes in the thyroid stimulating hormone content of the pituitary gland of female rats during the oestrous cycle and in pregnancy.

Marked changes occur in the activity of the thyroid gland of the rat during the oestrous cycle in association with the neuroendocrine events leading to ovulation (Brown-Grant, 1966), and plasma thyroid stimulating hormone (TSH) levels are highest during early oestrus (Boccabella & Alger, 1967). In the present study, possible changes in pituitary TSH content were investigated. Rats showing regular 4-day cycles were killed at different stages of the cycle or during pregnancy (Day 0 is the day sperm were found in the vaginal smear). Pituitaries were homogenized in 0·9% (w/v) NaCl solution and stored at −20 °C until assayed by a modification (El Kabir, Benhamou-Glynn, Doniach & Roitt, 1966) of the McKenzie assay. In any one assay the responses to three or four doses of standard and eight to ten pituitaries, each at a single dilution such that the response fell within the limits of the standard curve, were measured

Further observations on the effect of synthetic thyrotropin-releasing hormone in man.

Synthetic thyrotrophin-releasing hormone (TRH) given intravenously in doses of 50 mug or more causes a significant rise in serum thyroid-stimulating hormone (TSH) levels but has no effect on serum growth hormone, plasma luteinizing hormone, or plasma 11-hydroxycorticosteroids under carefully controlled basal conditions.The peak TSH response to intravenous TRH occurs at 20 minutes. The mild and transient side effects, which occur only after intravenous TRH, include nausea, a flushing sensation, a desire to micturate, a peculiar taste, and tightness in the chest. There is considerable variability in response to a given dose of TRH in the same subject on different occasions and in different subjects. Oral administration of TRH in doses of 1 mg and above causes a rise in serum TSH, maximal at two hours, a consistent response being obtained at doses of 20 mg and above. A rise in serum protein-bound iodine (P.B.I.) follows that of TSH, a consistent response being observed at 40-mg doses of TRH orally. Measurements of serum TSH after intravenous administration of TRH or of serum TSH or serum P.B.I. after oral TRH should prove useful tests of pituitary TSH reserve.

The influence of methallibure (ICI 33,828) on serum and pituitary thyroid-stimulating hormone levels in the rat.

SUMMARY The effect of methallibure (ICI 33,828), a non-steroidal pituitary inhibitor, on serum and pituitary thyroid-stimulating hormone (TSH) levels has been investigated. A biphasic action of the drug on serum TSH levels was observed, the greatest falls occurring with the lowest doses (2mg/day). Increasing dose and period of administration induced progressive decreases in pituitary TSH content. These results are interpreted in terms of three actions on the thyroid—pituitary system: (1) inhibition of the release of TSH from the pituitary, (2) inhibition of TSH synthesis evident only at higher doses, and (3) a thyroid-blocking action, which is also only observed at the higher dose levels, with consequent pituitary stimulation via the thyroid—pituitary feedback mechanism. Effects upon body weight and weight of endocrine organs are reported, that upon the seminal vesicles being the most marked.