Enlargement of the sella turcica in some patients with long standing untreated endemic cretinism, Serum TSH, alpha, TSH-beta, and prolactin responses to TRH

Diurnal thyrotropin secretion in short-term profound primary hypothyroidism: does it ever persist?

Circulating serum thyrotropin (TSH) levels in euthyroid humans show a definite circadian variation, which is maintained in both mild hyperthyroidism and mild hypothyroidism. Yet conflicting data exist with regard to whether this variation persists in at least some patients with severe primary hypothyroidism. We, therefore, studied the diurnal variation in serum TSH in 10 patients (age range 20 to 84 years) with a history of thyroid failure due to prior total thyroidectomy and radioiodine (RAI) ablative treatment performed for thyroid cancer after short-term discontinuation of thyroid hormone (TH) therapy. Serum TSH was measured hourly for a 24-hour period. All data were normalized by converting the TSH values to a percentage (%), designating the 11:00 hour value as 100% (baseline). The average serum TSH levels were markedly elevated in all patients. There was no statistically significant difference between the TSH % values at any time during the 24-hour period when compared with baseline. Further, cosine regression analysis showed absence of rhythmicity in TSH % values over time; notably, no patient showed a variation in TSH % values > or = 15% of baseline. In conclusion, diurnal rhythmicity in serum TSH levels was abolished in a uniform cohort of patients with short-term severe primary hypothyroidism. We speculate that the complete lack of peripheral negative feedback input to the hypothalamus or pituitary or both may override the central rhythm-sustaining influences on TSH secretion.

Endemic goitre--iodine deficiency disorders

Endemic goitre occurs when the prevalence of thyroid enlargement in the population of an area exceeds 10%. With few exceptions its cause is iodine deficiency superimposed on other goitrogenic factors normally present and responsible for sporadic goitre. Iodine deficiency causes significant health problems and so, the term iodine deficiency disorders (IDD) has been introduced. The earliest sign of IDD is goitre, but these disorders also include cretinism, neonatal hypothyroidism and congenital defects, as well as retardation of mental and physical development etc. IDD are a worldwide problem: WHO estimates that substantially more than 800 million people are at risk and more than 190 millions suffer from IDD; over 3 million people have cretinism and in the largest and worst affected areas many millions suffer from mental and physical developmental defects. IDD can be totally eliminated by prophylaxis using iodine administered in salt, oil or some other vehicle. Problems over preventing iodine deficiency relate to difficulties in the handling and distribution of the iodized vehicle in some parts of the world and on the political will to introduce preventive schemes. In only a very few areas does the presence of goitrogenic agents in the environment cause endemic goitre despite adequate iodine supply. In a limited number of places excessive iodine from seaweed used as staple food results in endemic goitre.

Pituitary-thyroid relationships in hypothyroidism

The pituitary gland is undoubtedly a target for thyroid hormone. It is the tissue with the highest density of T3 receptors and contains Type II T4 5' deiodinase which are involved in the secretion of TSH and other pituitary hormones. In the case of TSH, this knowledge has resulted in a better understanding of a number of conditions such as the adaptation to reduced thyroid reserve, and in an improvement in the way we treat hypothyroidism. But the pituitary being a target tissue for thyroid hormones has additional consequences. A direct effect of T3 on various secretions of the gland has been documented, and in the case of other pituitary hormones, directly or indirectly, thyroid hormone has some effect. T3 has a broad spectrum of metabolic and physiological effects that may, by themselves, account for a large proportion of the variability of the clinical presentation of hypothyroidism. The multiplicity of pituitary hormones and the multiplicative action of these various hormones through their target glands make the pituitary gland another key element in the variability of clinical manifestations of hypothyroidism. While this variability poses a diagnostic challenge in that hypothyroidism should be considered in a large number of conditions, the abnormalities resulting from the lack of thyroid hormone in the pituitary are equally challenging from a therapeutic point of view, since they respond promptly to the correction of the hypothyroidism. We should try to identify and treat those that are both life-threatening and not corrected rapidly by the administration of thyroid hormone.

Decreased growth hormone (GH) response to oral clonidine in endemic cretinism: effect of L-T3 therapy

As GH secretion is dependent upon thyroid hormone availability, the GH responses to clonidine (150 micrograms/m2) and the TSH and PRL response to TRH were studied in eight endemic (EC) cretins (3 hypothyroid, 5 with a low thyroid reserve) before and after 4 days of 100 micrograms of L-T3. Five normal controls (N) were also treated in similar conditions. Both groups presented a marked increase in serum T3 after therapy (N = 515 +/- 89 ng/dl; EC = 647 +/- 149 ng/dl) followed by a decrease in basal and peak TSH response to TRH. However, in the EC patients an increase in serum T4 levels and in basal PRL and peak PRL response to TRH after L-T3 therapy was observed. One hypothyroid EC had a markedly elevated PRL peak response to TRH (330 ng/dl). There were no significant changes in basal or peak GH values to treatment with L-T3 in normal subjects. In the EC group the mean basal plasma GH (2.3 +/- 1.9 ng/ml) significantly rose to 8.8 +/- 3.2 ng/ml and the mean peak response to clonidine (12.7 +/- 7.7 ng/ml) increased to 36.9 +/- 3.1 ng/ml after L-T3. Plasma SM-C levels significantly increased in N from 1.79 +/- 0.50 U/ml to 2.42 +/- 0.40 U/ml after L-T3 (p less than 0.01) and this latter value was significantly higher (p less than 0.05) than mean Sm-C levels attained after L-T3 in the EC group (respectively: 1.14 +/- 0.59 and 1.78 +/- 0.68 U/ml). These data indicate that in EC the impaired GH response to a central nervous system mediated stimulus, the relatively low plasma Sm-C concentrations, and the presence of clinical or subclinical hypothyroidism may contribute to the severity of growth retardation present in this syndrome.