Thyrotropin-releasing hormone and the pituitary. New insights into the mechanism of stimulated secretion and clinical usage

Unmet needs of patients with narcolepsy: perspectives on emerging treatment options

The treatment options currently available for narcolepsy are often unsatisfactory due to suboptimal efficacy, troublesome side effects, development of drug tolerance, and inconvenience. Our understanding of the neurobiology of narcolepsy has greatly improved over the last decade. This knowledge has not yet translated into additional therapeutic options for patients, but progress is being made. Some compounds, such as histaminergic H3 receptor antagonists, may prove useful in symptom control of narcolepsy. The prospect of finding a cure still seems distant, but hypocretin replacement therapy offers some promise. In this narrative review, we describe these developments and others which may yield more effective narcolepsy treatments in the future.

Hypopituitarism in childhood

BACKGROUND

Childhood hypopituitarism may be present at birth or may be acquired. Early diagnosis and treatment promote the best possible outcomes.

METHODS

The anatomy, etiologies, clinical presentation, diagnostic testing strategies, and current treatments relevant to childhood hypopituitarism are reviewed.

RESULTS

Children with congenital hypopituitarism may present with life-threatening hypoglycemia, abnormal serum sodium concentrations, shock, microphallus in males, and, only later, growth failure. Causes of congenital hypopituitarism include septo-optic dysplasia, other midline syndromes, and mutations of transcription factors involved in pituitary gland development. Children with acquired hypopituitarism typically present with growth failure and may have other complaints depending on the etiology and the extent of missing pituitary hormones. Acquired hypopituitarism may result from tumors (most commonly craniopharyngioma), radiation, infection, hydrocephalus, vascular anomalies, and trauma.

CONCLUSIONS

An MRI of the head is critical in determining the etiology. Testing for pituitary hormone deficiencies is undertaken along with appropriate hormonal replacement and, in some cases, direct treatment of the cause of the hypopituitarism. All children with hypopituitarism require coordination of medical care by a pediatric endocrinologist and, when older, transition to the care of an internist endocrinologist. Referrals to a reproductive endocrinologist may be required as fertility issues arise.

Post-herpes encephalitic anterior pituitary insufficiency with hypothermia and hypotension

A 49-year-old man with herpes simplex encephalitis at age 22 was admitted with hypotension (90/60 mm Hg) and hypothermia (33.7 degrees C). His blood pressure was 80-90/50-60 mm Hg, with temperatures averaging 35 degrees C, for at least 3 years before admission. Evaluation of his hypothermia and hypotension revealed a low free triiodothyronine, low normal thyrotropin, luteinizing hormone < 2 mIU/L, follicle stimulating hormone <3 mIU/L, and low testosterone of 1.39 ng/dL. A baseline cortisol of 13.9 microg/dL was stimulated to 41.8 microg/dL with corticotropin, indicating he had partial anterior hypopituitarism with an intact pituitary-adrenal axis. Posterior pituitary function was normal. MRI revealed a "bright" posterior pituitary on a T1-weighted image, further indicating a normal posterior pituitary. Extensive decreased T1-weighting on MRI in the right and left temporal lobes was consistent with encephalomalacia. With thyroid hormone replacement, his blood pressure increased to 110/70 mm Hg with a temperature of 37 degrees C.

Thyroid-stimulating hormone and total thyroxine concentrations in euthyroid, sick euthyroid and hypothyroid dogs

Canine thyroid-stimulating hormone (cTSH) was measured in a variety of clinical cases (n = 72). The cases were classified as euthyroid, sick euthyroid, hypothyroid or hypothyroid on nonthyroidal therapy on the basis of their history, clinical signs, laboratory results (including total thyroxine concentrations and, where indicated, thyroid-releasing hormone [TRH] stimulation tests) and response to appropriate therapy. Additional samples were taken during some of the TRH stimulation tests to measure the response of cTSH concentrations following TRH administration. A reference range (0 to 0.41 ng/ml) was calculated from the basal concentrations of cTSH in a group of 41 euthyroid dogs. Six of nine cases of confirmed hypothyroidism had basal cTSH concentrations above the reference range, whereas the remainder were within the normal range. One of these three remaining cases was a pituitary dwarf and did not show a rise in cTSH concentration following TRH stimulation. In contrast, only one of a group of six hypothyroid dogs that had been on non-thyroidal treatment within the previous four weeks had increased concentrations of basal cTSH. This study also found that five of a group of 16 dogs with sick euthyroid syndrome had increased cTSH concentrations. It was concluded that cTSH measurements are a useful additional diagnostic test in cases of suspected hypothyroidism in dogs but that dynamic testing is still required to confirm the diagnosis of hypothyroidism.

Effects of thyrotropin-releasing hormone and its analogs on daytime sleepiness and cataplexy in canine narcolepsy

The therapeutic potential of thyrotropin-releasing hormone (TRH) and TRH analogs in narcolepsy, a sleep disorder characterized by abnormal rapid eye movement (REM) sleep and daytime sleepiness, was examined using the canine model. The effects of TRH and the biologically stable TRH analogs CG3703, CG3509, and TA0910 on daytime sleep and cataplexy, a symptom of abnormal REM sleep, were assessed using polysomnographic recordings and the food elicited cataplexy test (FECT), respectively. CG3703 (100 and 400 microg/kg, i.v.) and TA0910 (100 and 400 microg/kg, i.v.) significantly increased wakefulness and decreased sleep in narcoleptic canines, whereas TRH (400 and 1600 microg/kg, i.v.) had no significant effect. TRH (25-1600 microg/kg, i.v.) and all three TRH analogs, CG3703 (6. 25-400 microg/kg, i.v., and 0.25-16 mg/kg, p.o.), CG3509 (25-1600 microg/kg, i.v.), and TA0910 (25-1600 microg/kg, i.v.), significantly reduced cataplexy in canine narcolepsy. These compounds did not produce any significant side effects during behavioral assays, nor did they alter free T3 and T4 levels in serum even when used at doses that completely suppressed cataplexy. Although more work is needed to establish the mode of action of TRH analogs on alertness and REM sleep-related symptoms, our results suggest a possible therapeutic application for TRH analogs in human sleep disorders.

Fast, short-term response to TRH stimulation in geriatric patients and its clinical importance

In 52 geriatric patients, average age of 74 years (range from 65 to 89) suffering from generalised arteriosclerotic disease and hospitalized at the Clinical Institute for Geriatrics, we found normal thyroid gland function, in basal condition. Our data showed that 19.42% of the patients had low triiodothyronine (T(3)) concentrations. Basal serum T(3) level was higher (P < 0.01) in males (1.88 +/- 0.44 nmol/l) in comparison to values in females (1.75 +/- 0.28). Serum thyroxine (T(4)) level was lower in males (P < 0.01), but the concentrations of thyreo stimulating hormone (TSH) was lower in females. The value of thyroid reserve in elderly people, estimated by measuring T(3) and T(4) incretion shortly after thyreotropin releasing hormone (TRH) was done, is sufficient to maintain cuthyroid function, although it is, as a whole, significantly lower if compared with T(4) and T(3) response after TRH tests in middle-aged subjects (n = 26), acting as a control group (P < 0.001). Thyroxine excretion from follicular cells in elderly female patients was faster and amplitude was higher, up to the maximal possible level 93 nmol/l (+/- SD) in 25 min after an injection of TRH. In elderly male patients the maximum of T(4) excretion was 78 nmol/l (P < 0.01) at 60 min.

The loss of 45Ca2+ associated with prolactin release from the tilapia (Oreochromis mossambicus) rostral pars distalis

The relationship between tritium 3H-labeled prolactin (PRL) release and the loss of tissue-associated 45Ca2+ was examined in the tilapia rostral pars distalis (RPD) using perifusion incubation under conditions which inhibit or stimulate PRL release. Depolarizing [K+] (56 mM) and hyposmotic medium (280 mOsmolal) increased both the release of [3H]PRL and the loss of 45Ca2+. The responses to high [K+] were faster and shorter in duration than those produced by reduced osmotic pressure. The depletion of Ca2+ from the incubation medium with 2 mM EGTA suppressed the [3H]PRL response evoked by high [K+] or reduced osmotic pressure. Exposing the tissues to Ca(2+)-depleted medium in the absence of high [K+] or reduced osmotic pressure produced a sharp, but brief, increase in 45Ca2+ loss. Cobalt (10(-3) M), a competitive inhibitor of calcium-mediated processes, inhibited the [3H]PRL response to hyposmotic medium and to high [K+]. Cobalt also diminished the increased loss of 45Ca2+ evoked by exposure to reduced osmotic pressure, but was ineffective in altering responses to high [K+]. Methoxyverapamil (D600; 10(-5) M), a blocker of certain voltage-sensitive Ca2+ channels, did not alter either the [3H]PRL or the 45Ca2+ responses to high [K+] and reduced osmotic pressure. Taken together with our earlier studies, the present findings suggest that exposure to high [K+] or hyposmotic medium produces rapid changes in the Ca2+ metabolism of the tilapia RPD that are linked to the stimulation of PRL secretion. Nevertheless, the increased 45Ca2+ loss, but not [3H]PRL release, upon exposure to Ca(2+)-depleted media suggests that Ca2+ loss may not always reflect intracellular events that lead to PRL release.

Is idiopathic atrial fibrillation caused by occult thyrotoxicosis? A study of one hundred consecutive patients with atrial fibrillation

In this study we evaluated the possible relationship between idiopathic atrial fibrillation and occult thyrotoxicosis, diagnosed by lack of response of thyroid stimulating hormone to administration of thyrotropin releasing hormone. Three groups were compared: 25 patients with idiopathic atrial fibrillation; 52 with cardiovascular atrial fibrillation; and 27 with sinus rhythm. Patients were excluded with any clinical evidence of thyrotoxicosis or with elevated serum FT4 level, as well as those with diseases or on medications known to be associated with a diminished response to administration of the releasing hormone. A flat test (lack of response) was found in only 4% of the patients. There was no significant difference among the 3 groups. Based on these data we believe that there is no relationship between idiopathic atrial fibrillation or any other type of atrial fibrillation and occult thyrotoxicosis.

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)

Effects of thyroid hormones (T4,T3), bromocriptine and Triac on inappropriate TSH hypersecretion

Inappropriate TSH hypersecretion was diagnosed in a 38-year-old woman (case 1) and in a 38-year-old man (case 2). Both of them had earlier been treated by ablative therapy for hyperthyroidism. The present diagnosis was based on elevated basal serum TSH levels despite elevated serum free thyroid hormone levels. Both of them had exaggerated TSH responses to TRH (peak value 240 mU/l in case 1 and 408 mU/l in case 2). Their albumin and prealbumin levels were normal. The serum TBG level was normal in case 1 but was elevated in case 2. Serum levels of alpha-subunits of TSH, and pituitary CT scans were normal. Despite mild clinical hyperthyroidism, peripheral indices of thyroid hormone action were normal. They had also relatives with apparent resistance to thyroid hormones. In view of the possibility that prolonged pituitary thyrotrophic stimulation is detrimental, various therapeutic approaches to suppress TSH levels were tried. Both T3 and T4 treatments lowered serum TSH levels, but were poorly tolerated. Acute administration of L-dopa or bromocriptine reduced serum TSH levels, but this was not seen during long-term therapy. TRIAC treatment lowered serum TSH levels, and the drug was well tolerated. Serum TSH responses to TRH were not blunted during T3, T4 or TRIAC treatments. Somatostatin also reduced serum TSH levels, but did not potentiate the effect of low dose T3 therapy. Our results suggest that the patients had unbalanced pituitary and peripheral thyroid hormone resistance, predominantly at the pituitary level. Of the drugs studied, TRIAC seemed to be the most suitable therapy.

Serum thyrotrophin in patients with destructive pituitary lesions assessed by a sensitive immunoradiometric assay

Using a sensitive immunoradiometric assay, serum concentrations of thyrotrophin (TSH) were measured both basally and after intravenous thyrotrophin releasing hormone (TRH) in two groups of patients with destructive pituitary lesions. Group A patients, who had more extensive pituitary hormone deficiencies and biochemical evidence of hypothyroidism at the time of study, had significantly lower mean serum TSH concentrations than the group B patients who were biochemically euthyroid. Basal TSH was below the lowest value seen in a control group of normal subjects in 5 of the 8 group A patients. In all patients TSH increased significantly after injection of TRH; the peak values being significantly correlated with the basal. In contrast to recent studies, which have emphasised other mechanisms in the production of secondary hypothyroidism, our data suggest that inadequate production of immunoassayable TSH is a common cause of secondary hypothyroidism in patients with destructive pituitary disease. Little, if any, additional clinical information is to be gained by measurement of TSH after TRH stimulation in this circumstance.

Ultrasensitive assay of thyroid stimulating hormone in patients with acute non-thyroidal illness

Serum TSH in critically ill euthyroid patients is generally within the normal range when measured with conventional radioimmunoassays. Sensitive immunoradiometric assays allow detection of low levels of serum TSH. We assessed this method in a prospective study of 34 euthyroid patients admitted to our critical care unit. Serum TSH ranged from 0.12 to 3.60 mU/l and was significantly lower for the whole group than in the controls (P less than 0.001), as also were serum total T4 and T3 values (P less than 0.001). However, 21 patients had a serum TSH within the normal range (group 1) and 13 patients (33%) had a serum TSH less than 0.40 mU/l (group 2). The two groups did not differ in age, sex, type and severity of illness, outcome, and serum T4 and T3 levels. However, the magnitude of TSH increase from the baseline value after the i.v. injection of 200 micrograms of TRH, assessed by the 30 min TSH/basal TSH ratio was significantly higher in group 2 (P less than 0.05). These results suggest that a substantial proportion of patients with acute illness have a clearly low serum TSH, unaccounted for by age, sex, type or quantified severity of illness, serum T4 and T3 levels. In addition, the secretory capacity of the pituitary to exogeneous TRH is significantly enhanced in those patients with low basal TSH.