Nociceptin stimulates thyrotropin secretion in rats

Effects of nociceptin on thyrotropin (TSH) and thyrotropin-releasing hormone (TRH) secretion in rats were studied. Nociceptin (150 microgram/kg) was injected intravenously and rats were serially decapitated after the injection. The effects of nociceptin on TRH release from the hypothalamus and TSH release from the anterior pituitary in vitro were also investigated. TRH and thyroid hormones were measured by individual radioimmunoassays. TSH was determined by enzyme immunoassay. TRH contents in the hypothalamus decreased significantly after nociceptin injection, whereas plasma TRH concentrations showed no changes. Plasma TSH concentrations increased significantly in a dose-related manner. The TRH release from the hypothalamus was enhanced significantly in a dose-related manner with the addition of nociceptin. The TSH release from the anterior pituitary in vitro was not affected by the addition of nociceptin. The plasma thyroxine and 3,3',5-triiodothyronine levels did not change significantly after nociceptin administration. The inactivation of TRH by plasma or hypothalamus in vitro after nociceptin injection did not differ from that of controls. The findings suggest that nociceptin acts on the hypothalamus to stimulate TRH and TSH secretion.     

Effects of orexin A on thyrotropin-releasing hormone and thyrotropin secretion in rats.

Effects of orexin A on secretion of thyrotropin-releasing hormone (TRH) and thyrotropin (TSH) in rats were studied. Orexin A (50 microg/kg) was injected iv, and the rats were serially decapitated. The effects of orexin A on TRH release from the rat hypothalamus in vitro and on TSH release from the anterior pituitary in vitro were also investigated. TRH and thyroid hormone were measured by individual radioimmunoassays. TSH was determined by the enzyme-immunoassay method. The hypothalamic TRH contents increased significantly after orexin A injection, whereas its plasma concentrations tended to decrease, but not significantly. The plasma TSH levels decreased significantly in a dose-related manner with a nadir at 15 min after injection. The plasma thyroid hormone levels showed no changes. TRH release from the rat hypothalamus in vitro was inhibited significantly in a dose-related manner with the addition of orexin A. TSH release from the anterior pituitary in vitro was not affected with the addition of orexin A. The findings suggest that orexin A acts on the hypothalamus to inhibit TRH release.     

Increase of thyrotropin response to thyrotropin-releasing hormone (TRH) and TRH release in rats during pregnancy

Regulation of thyrotropin (TSH) release by thyrotropin releasing hormone (TRH) in the anterior pituitary gland (AP) of pregnant rats was studied. The pregnant (day 7, 14, and 21) and diestrous rats were decapitated. AP was divided into 2 halves, and then incubated with Locke's solution at 37 degrees C for 30 min following a preincubation. After replacing with media, APs were incubated with Locke's solution containing 0, or 10 nM TRH for 30 min. Both basal and TRH-stimulated media were collected at the end of incubation. Medial basal hypothalamus (MBH) was incubated with Locke's medium at 37 degrees C for 30 min. Concentrations of TSH in medium and plasma samples as well as the cyclic 3':5' adenosine monophosphate (cAMP) content in APs and the levels of TRH in MBH medium were measured by radioimmunoassay. The levels of plasma TSH were higher in pregnant rats of day 21 than in diestrous rats. The spontaneous release of TSH in vitro was unaltered by pregnancy. TRH increased the release of TSH by AP, which was higher in pregnant than in diestrous rats. Maternal serum concentration of total T3 was decreased during the pregnancy. The basal release of hypothalamic TRH in vitro was greater in late pregnant rats than in diestrous rats. After TRH stimulation, the increase of the content of pituitary cAMP was greater in late pregnant rats than in diestrus animals. These results suggest that the greater secretion of TSH in pregnant rats is in part due to an increase of spontaneous release of TRH by MBH and a decrease of plasma thyroid hormones. Moreover, the higher level of plasma TSH in rats during late pregnancy is associated with the greater response of pituitary cAMP and TSH to TRH.     

Inhibitory effects of cysteamine on neuroendocrine function

The action of cysteamine on anterior pituitary hormone secretion was studied in vivo using conscious, freely moving male rats and in vitro using anterior pituitary cells in monolayer culture. Administration of 500 micrograms cysteamine into the lateral cerebral ventricles of normal rats caused the complete inhibition of pulsatile GH secretion for a minimum of 6 h. This treatment also significantly decreased plasma concentrations of LH for at least 6 h in orchiectomized rat, TSH in short-term (0.5 month) thyroidectomized rats, and PRL in long-term (6 months) thyroidectomized rats. The in vivo stimulation of GH, LH, TSH and PRL with their respective releasing hormones 60 min after administration of cysteamine was not different from the response observed in rats pretreated with saline except for PRL where cysteamine pretreatment significantly inhibited the expected PRL increase. In vitro, 1 mM cysteamine decreased basal and TRH stimulated PRL release while not affecting basal or stimulated GH, LH, TSH and ACTH secretion. These data demonstrate the dramatic and wide-ranging effects of cysteamine on anterior pituitary hormone secretion. This action appears to be mediated through hypothalamic pathways for GH, LH and TSH and through a pituitary pathway for PRL.     

Effects of ether and pentobarbital anesthesia on thyroid function in the rat.

Studies were performed to examine the effect of two anesthetic agents, ether and pentobarbital, on the hypothalamic-pituitary-thyroid function in vivo. In non-anesthetized animals, plasma thyrotropin (TSH) increased rapidly from basal values of 0.1, a peak of 0.49 microng/ml, 25 min after exposure to the cold. Anesthesia with ether during exposure to the cold completely prevented the rise in TSH. During pentobarbital anesthesia, the rise in TSH after exposure to cold was reduced by more than 90%. Even a three minute period of ether anesthesia prior to cold exposure reduced the peak response to cold as well as delayed this response when compared to the untreated group. During two hours of anesthesia with ether, the TSH concentration declined in animals which were fed a low iodine diet at essentially the same rate as in animals on the same diet given an injection of 3 microng of triiodothyronine. Pentobarbital did not suppress TSH at room temperature. The release of thyrotropin after injection of synthetic thyrotropin-releasing hormone (TRH) was greater in animals anesthetized with pentobarbital than in controls and was slightly reduced in ether-anesthetized animals. This difference was observed when thyrotropin was given intraperitoneally or intravenously and the slope of the dose-response curves to TRH showed a flattening of the curve of rats treated with ether and a steeper slope of response in animals anesthetized with pentobarbital. We conclude that pentobarbital inhibited TSH response to cold but did not reduce the resting levels. Ether inhibited the rise of TSH in the cold and lowered the basal levels of TSH in animlas at room temperature. Pentobarbital increased the response to TRH and ether may have reduced the response to TRH.     

Effects of histamine and related compounds on thyrotropin secretion in rats

The effects of histamine (HA) and related compounds on thyrotropin-releasing hormone (TRH) and thyrotropin (TSH) secretion in rats were studied. Histidine (1.0 g/kg), HA (5.0 mg/kg) or histamine antagonists mepyramine (MP) (100 mg/kg) or famotidine (FA) (5.0 mg/kg) were injected intraperitoneally, and the rats were decapitated at various intervals after the injection. The hypothalamic immunoreactive TRH (ir-TRH) content increased significantly after histidine or HA injection, decreased significantly after FA injection, but was not changed by MP. The plasma ir-TRH concentration did not change significantly after injection of these drugs. The plasma TSH levels decreased significantly in a dose-related manner after histidine or HA injection and increased significantly in a dose-related manner after FA injection. The plasma thyroid hormone levels showed no changes. In the FA-pretreated group, the inhibitory effect of histidine or HA on TSH levels was prevented, but not in the MP-pretreated group. The plasma ir-TRH and TSH responses to cold were inhibited by histidine or HA and enhanced by FA. The plasma TSH response to TRH was inhibited by histidine or HA and enhanced by FA. The inactivation of TRH immunoreactivity by hypothalamus or plasma in vitro after histidine, HA, MP or FA was not different from that of the control. These findings suggest that histamine may act both on the hypothalamus and the pituitary to inhibit TRH and TSH release, and that its effects may be mediated via H2-receptor.     

Thyrotropin secretagogues reduce rat pituitary neuromedin B, a local thyrotropin release inhibitor

Neuromedin B (NB), a bombesin-like peptide, highly concentrated in rat pituitary gland, has been shown to act as an autocrine/paracrine inhibitor of thyrotropin (TSH) release. Here it is shown that a single injection of thyrotropin-releasing hormone (TRH, 1.5 microg/animal, ip), the most important stimulator of thyrotropin secretion, induced approximately 35%-45% decrease in pituitary NB content in rats, as well as an important decrease in NB mRNA at 15 and 30 min (P < 0.05). Acute cold exposure, which induced higher serum TSH with a peak at 30 min, was associated with progressive decrease in pituitary NB, starting at 15 min although only reaching statistical significance after 2 hr (P < 0.05). Although not involved in the early peak, the decrease in NB may be contributing to maintenance of higher serum TSH in cold-exposed animals compared with those at room temperature. Fed rats, 2 hr after being subcutaneously injected with mouse recombinant leptin (8 microg /100 g body wt), showed a x2 increase in serum TSH and 38% reduction in pituitary NB (P < 0.05). In conclusion, TRH and leptin rapidly decreased pituitary NB and it is first proposed that the reduction of the inhibitory tonus of NB on TSH release will ultimately contribute to the amplification of TSH secretion elicited by TSH secretagogues.     

The pituitary-thyroid axis in patients with pituitary disorders

The pituitary-thyroid axis of 12 patients, exposed to transsphenoidal pituitary microsurgery because of nonfunctioning adenomas (6), prolactinomas (3) and craniopharyngioma (1), or to major pituitary injury (1 apoplexy, 1 accidental injury), was controlled more than 6 months following the incidents. The patients did not receive thyroid replacement therapy and were evaluated by measurement of the serum concentration of thyroxine (T4), 3,5,3'-triiodothyronine (T3), 3,3',5'-triiodothyronine (rT3), T3-resin uptake test and thyrotropin (TSH, IRMA method) before and after 200 micrograms thyrotropin releasing hormone (TRH) iv. The examination also included measurement of prolactin (PRL) and cortisol (C) in serum. Apart from 1 patient with pituitary apoplexy all had normal basal TSH levels and 9 showed a significant TSH response to TRH. Compared to 40 normal control subjects the 12 patients had significantly decreased levels of T4, T3 and rT3 (expressed in free indices), while the TSH levels showed no change. Five of the patients, studied before and following surgery, had all decreased and subnormal FT4I (free T4 index) after surgery, but unchanged FT3I and TSH. The levels of FT4I were positively correlated to both those of FT3I and FrT3I, but not to TSH. The TSH and thyroid hormone values showed no relationship to the levels of PRL or C of the patients exposed to surgery. It is concluded that the risk of hypothyroidism in patients exposed to pituitary microsurgery is not appearing from the TSH response to TRH, but from the thyroid hormone levels.     

Effects of leucine-enkephalin on hypothalamic-pituitary-thyroid axis in rats

Basal thyrotropin (TSH) levels in plasma and the TSH response to thyrotropin-releasing hormone (TRH) were inhibited after Leucine-enkephalin (L-EK) administration iv in rats. TRH and TSH responses to cold were inhibited after L-EK administration. In the L-DOPA, haloperidol or 5-hydoxytryptophan-treated rats, the inhibitory effect of L-EK on TSH release was restored. Findings suggested that L-EK acted both the hypothalamus and pituitary. Its inhibitory effects on TRH and TSH release at least partially mediated by interaction with amines in the central nervous system.     

Effect of sulpiride on serum growth hormone and prolactin concentrations following L-DOPA administration in man.

The effect of chronic administration of sulpiride on serum human growth hormone (hGH), prolactin and thyroid stimulating hormone (TSH) was examined in 6 normal subjects. Sulpiride was given orally at a dose of 300 mg (t.i.d.) for 30 days. Sulpiride raised serum prolactin levels in all subjects examined. In addition, sulpiride suppressed hGH release induced by L-dopa, although the basal hGH level was not changed. Sulpiride treatment appeared to antagonize partially the inhibitory effect of L-dopa on prolactin release. Following thyrotropin-releasing hormone (TRH) injection, the percent increment in prolactin levels from the baseline in sulpiride-treated subjects was less than in controls without sulpiride. In contrast, both the basal and TRH-stimulated TSH levels were not influenced by sulpiride. These observations suggest that sulpiride suppresses L-dopa-induced hGH release and stimulates prolactin release, presumably by acting against the dopaminergic mechanism either on the hypothalamus or on the pituitary. The decreased prolactin response to TRH after sulpiride treatment may indicate a diminished reserve capacity in pituitary prolactin release.     

Variations in the response of enzyme-dissociated rat pituitary cells to thyrotropin releasing hormone.

While exploring the interaction between thyrotropin releasing hormone (TRH) and normal rat anterior pituitary cells in monolayer culture we observed that cells dissociated with the use of trypsin did not respond to TRH with an increase in either TSH or prolactin (PRL) release. The dissociated cells were cultured for 3 days, then washed to remove serum proteins and exposed to 10^?6M TRH for 3 hours. TSH and PRL secretion from stimulated and unstimulated cultures was determined by radio-immunoassay and normalized using cell protein. When such trypsin-dissociated cells were exposed to 0.5 mM dibutyryl cyclic AMP the release of both TSH and PRL doubled indicating that the intracellular secretory machinery was functional and that the block to TRH was proximal to the formation of cyclic AMP and presumably at the level of a TRH surface receptor. Previous studies have shown that such trypsin-dissociated cells respond to LHRH and a crude hypothalamic extract with a dose dependent increase in LH, FSH and ACTH release. This rules out a non-specific effect of trypsin. When pituitary cells were dissociated with a non-trypsin technique, the unstimulated release of both TSH and PRL was comparable to that found with the trypsin-dissociated cultures. However, these cultures did respond to TRH with an increase in TSH release although again no effect was seen with PRL. The susceptibility of the cells to trypsin suggests the possibility that a protein moiety may be closely associated with the function of the receptor.     

Effect of 3-hydroxy-4-1(H)pyridone on the basal and thyrotropin-releasing hormone-stimulated release of thyroid-stimulating hormone from perifused anterior pituitary fragments

This study investigated the direct effect of 3-hydroxy-4-1(H)-pyridone (DHP), the breakdown product of mimosine in the rumen, on thyroid-stimulating hormone (TSH) secretion by perifusion of rat anterior pituitary fragments. During a 2-h perifusion with thyrotropin-releasing hormone (TRH), the total release of TSH increased linearly (P less than 0.05, r = 0.966) with increasing concentration of TRH from 1 to 100 ng/ml. The release was maximal at 100 ng/ml. There were no differences in total basal TSH release among control and DHP-treated pituitary fragments. DHP at concentrations of 1, 10, and 100 micrograms/ml had no significant effect on the TSH response to TRH. However, DHP at the concentration of 1 mg/ml significantly suppressed the TSH response to TRH administered continuously or as a 10-min pulse. These results suggest that DHP modulates the pituitary thyrotroph's response to TRH.     

Unchanged response to stimulation of pituitary hormone release after serial UV irradiation in men

A group of 24 healthy young men were evaluated before and after serial suberythematous ultraviolet (UV) radiation: group I, control (no irradiation); groups II and III, 12 radiations in 4 weeks with two different spectra (both containing UV-B). Before the first and 2 days after the last exposure all the volunteers were given an intravenous injection of thyrotropin releasing hormone (TRH, protirelin 0.2 mg) and luteinizing hormone releasing hormone (LH-RH, gonadorelin 0.1 mg). The serum concentrations of TSH, follicle stimulating hormone, LH and prolactin were measured at 0, 20, 30, 45 and 60 min by radioimmunoassay. Neither basal nor stimulated levels of the pituitary hormones showed significant changes after UV radiation. The results showed that exposure to suberythematous doses of UV did not influence the regulation of pituitary hormones in these healthy individuals. Accepted: 24 October 1996     

Effects of various drugs on thyrotropin secretion in rats

The effects of alpha-neoendorphin, kyotorphin, melatonin or diphenylhydantoin (DPH) on thyrotropin-releasing hormone (TRH) and thyrotropin (TSH) release in rats were studied. alpha-neoendorphin (1.0 mg/kg), kyotorphin (1.0 mg/kg), melatonin (2.5 mg/kg) or DPH (75 mg/kg) was injected iv or ip, and the rats were serially decapitated. TRH, TSH and thyroid hormone were determined by radioimmunoassay. The hypothalamic immunoreactive (ir-TRH) contents decreased significantly after melatonin injection, but not after alpha-neoendorphin, kyotorphin or DPH. The plasma ir-TRH concentrations decreased significantly after DPH injection, but not after alpha-neoendorphin, kyotorphin or melatonin. The plasma TSH levels decreased significantly in a dose-related manner with a nadir at 10 min. after melatonin, at 30 min. after DPH and at 40 min. after alpha-neoendorphin or kyotorphin injection. The plasma thyroid hormone levels did not change significantly after these drugs injection. The plasma ir-TRH and TSH responses to cold were inhibited by these drugs, but the plasma TSH response to TRH was not influenced. In the L-DOPA- or 5-hydroxy-tryptophan (5-HTP)-pretreated group, the inhibitory effect of alpha-neoendorphin or kyotorphin on TSH levels was prevented, but not in the haloperidol- or para-chloprophenylalanine (PCPA)- pretreated group. In the haloperidol- or PCPA-pretreated group, the inhibitory effect of melatonin on TSH levels was prevented, but not in the L-DOPA- or 5-HTP-pretreated group. These drugs alone did not affect plasma TSH levels in terms of the dose used. The inactivation of TRH immunoreactivity by hypothalamus or plasma in vitro after these drugs injection did not differ from that of the control.(ABSTRACT TRUNCATED AT 250 WORDS)     

Abnormalities of the thyroid hormone negative feedback regulation of TSH secretion in spontaneously hypertensive rats.

Spontaneously hypertensive rats (SHR) are characterized by several neuroendocrine abnormalities including a chronic hypersecretion of thyrotropin (TSH) of unknown etiology. We hypothesized that the inappropriately high TSH secretion in SHR may be the result of an impaired thyroid hormone negative feedback regulation of hypothalamic thyrotropin-releasing hormone (TRH) and/or pituitary TSH production. To test this hypothesis, SHR or their normotensive Wistar-Kyoto (WKY) controls were treated with either methimazole (0.02% in drinking water) to induce hypothyroidism or administered L-thyroxine (T4) at a dose of 0.8 or 2.0 micrograms/100 g body weight/day to induce hyperthyroidism. All treatments were continued for 14 days after which animals were killed under low stress conditions. TSH concentrations in plasma and anterior pituitary tissue were 2-fold higher (P less than 0.01) in euthyroid SHR compared to WKY control rats while thyroid hormone (T3 and T4) levels were in the normal range. Hypothyroidism induced by either methimazole or thyroidectomy caused a significant (P less than 0.01) rise of plasma TSH levels in both WKY and SHR rats. However, relative to the TSH concentrations in control animals, the increase of plasma TSH in SHR was significantly blunted (P less than 0.01) in comparison to the WKY group. Hypothyroidism caused a significant depletion of TRH in stalk-median eminence (SME) tissue in both groups of rats. However, no differences between SHR and WKY rats were observed. The administration of thyroid hormone caused a dose dependent suppression of plasma TSH levels in both strains of rats. However, at both doses tested plasma TSH concentrations in SHR rats were significantly less suppressed (P less than 0.05) than those in WKY animals. Under in vitro conditions basal and potassium induced TRH release from SMEs derived from SHR was significantly (P less than 0.05) higher than that from WKY rats, whether expressed in absolute terms or as percent of content. These findings suggest that the thyroid hormone negative feedback regulation of TSH secretion may be impaired in SHR rats. Our data do not allow conclusions as to whether defects in the regulation of TSH production are located exclusively at the hypothalamic level. Since the overproduction of hypothalamic TRH and hypophysial TSH should lead to an increased thyroid hormone biosynthesis other defects in the hypothalamus-pituitary-thyroid-axis may contribute to the abnormal regulation of TSH secretion in SHR rats.     

Physiological regulation of thyrotropin

The pattern of TSH secretion in man in pulsatile in addition to the well known circadian variation. The mechanism triggering TSH pulses remains unclear to date. Infusions of somatostatin or dopamine rapidly lowering basal TSH levels without suppressing the pulsatile pattern suggest that an episodic disinhibition exerted by a physiological inhibitor is not a likely cause. On the same basis, thyroid hormones do not appear to be candidates, since they similarly inhibit basal TSH levels after a time lag of several hours but again do not suppress pulsatile release of the hormone. In contrast, bolus injections of dexamethasone completely abolish pulsatile release of TSH for several hours despite a normal sensitivity of the pituitary to exogenous TRH, suggesting a hypothalamic action of the drug. The hypothesis that pulsatile TSH release might be governed by a pulsatile mode of a hypothalamic stimulator is supported by the observation that an infusion of nifedipine, a calcium channel blocker, which in vitro selectively inhibits the TRH effect on TSH but not prolactin secretion, exerts a comparable effect when it is infused in vivo.     

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.     

A physiological role of E and F series prostaglandins in the regulation of TSH secretion

The regulation of TSH secretion by E1, E2, E1 alpha and F2 alpha prostaglandins was studied by means of a monolayer culture system of dispersed rat anterior pituitary cells which was appropriately responsive to TRH, T3 and SRIF. PGEs and Fs induced significant increases in basal TSH release of the order of 30% at 10(-9) or 10(-8) to 10(-5) or 10(-4) M. Only PGEs accentuated the TSH release induced by a half maximal dose of TRH (10(-9) M) of the order of 60% in a dose dependent manner (10(-9) to 10(-6) M of PGEs), whereas PGFs did not. SRIF (10(-8) or 10(-9) M) alone failed to alter basal TSH release but did completely inhibit the TSH response to TRH (10(-9) M). SRIF also significantly inhibited both the increase in basal TSH release and the accentuation of the TSH response to TRH induced by PGEs (10(-6) M) but did not diminish the enhancement of basal TSH release induced by PGFs (10(-6) M). 7-oxa-13-prostynoic acid (PY1), a prostaglandin antagonist, which can act as an agonist in some systems, itself exhibited agonistic properties of PGEs with respect to basal and TRH induced TSH release. PY1 failed to inhibit the TSH release induced by all PGs, but partially inhibited the accentuated TSH response to TRH induced by PGEs. Indomethacin, PG synthetase inhibitor, did not affect basal or TRH induced TSH release in our system. These data suggest that PGs of the E and F series probably modulate TSH release via different mechanisms and that the PGE effect on basal TSH release differs from its augmentation of TRH induced TSH response. It is speculated that these effects of PGs may have physiological significance.     

Thyrotropin-secreting pituitary adenoma: a case report.

We report a 44-year-old male with a thyrotropin (TSH)-secreting pituitary adenoma. Based serum free triiodothyronine (FT3, 12.1 pmol/l) and free thyroxine (FT4, 28 pmol/l) were increased with normal basal TSH (3.1 mU/l). There was impaired TSH response to thyrotropin releasing hormone (TRH) test. Serum TSH was suppressed to 59% of the basal level after oral administration of 1.4 mg 3,3'-5-triiodothyroacetic acid (triac), whereas no suppression was observed after 75 micrograms daily administration of triiodothyronine (T3). Serum concentrations of alpha-subunit of TSH (TSH-alpha) and TSH-alpha/TSH molar ratio were high, being 1.95 micrograms/l, and 4.4, respectively. Pituitary CT and MRI scan showed the presence of a macroadenoma in the anterior lobe of the pituitary gland. Histopathology of the excised pituitary confirmed the diagnosis of a TSH-producing adenoma. A positive correlation between TSH and FT3 (r = 0.66, P less than 0.01) or FT4 (r = 0.54, P less than 0.01) was observed in serial sera obtained before and after operation.     

Serum free thyrotropin subunit in congenital isolated thyrotropin deficiency

In two patients with congenital isolated thyrotropin (TSH) deficiency, serum TSH determined by a sensitive immunoradiometric assay (IRMA) was consistently undetectable. The basal levels of serum free TSH-alpha subunit (TSH-alpha) determined by a specific radioimmunoassay (RIA) were elevated in the hypothyroid state, and decreased to the undectable level during displacement therapy with thyroid hormone. The serum free TSH-alpha significantly increased following intravenous administration of thyrotropin releasing hormone (TRH). Serum free TSH-beta subunit (TSH-beta) was undectable. These findings suggest that TSH deficiency in this disease is not due to absence of thyrotroph in the pituitary gland or deficiency of TSH-alpha, but to abnormalities of the TSH-beta gene.