Somatostatin and the 24 h levels of serum TSH, T3, T4, and reverse T3 in normals, diabetics and patients treated for myxoedema

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Abstract
The influence of somatostatin on serum TSH [thyrotropin], 3,5,3''-triiodothyronine (T3), free T3, thyroxine (T4), free T4 and 3,3'',5''-triiodothyronine (reverse T3, rT3) was studied in 5 healthy young subjects, in 7 young patients with juvenile type diabetes on a fixed daily insulin dose and in 5 patients with treated myxoedema. Blood samples were taken hourly during a 24 h control period and during a 24 h somatostatin infusion period. Somatostatin infusion obliterated the night increase in serum TSH in all 3 groups. The lower daytime serum TSH was not altered by somatostatin. Somatostatin inhibits the TSH secretion induced by TRH [thyroliberin]. Together these findings suggest that the high TSH level at night is induced by a hypothalamic TRH surge, while the lower day level of TSH is quite independent of hypothalamic TRH. Somatostatin infusion was accompanied by a minor fall in serum T4 in all groups. In normal subjects and diabetics serum free T3 fell 23 .+-. 6% and 25 .+-. 6%, respectively, during 24 h somatostatin infusion. In patients with treated myxedema, serum free T3 was not significantly affected. These results are consistent with a decrease in thyroidal secretion during somatostatin infusion, as the cause for the fall in serum T3 in normal subjects and in patients with diabetes, while the monodeiodination of T4 and T3 in peripheral tissues is unaffected. An extrathyroidal effect of somatostatin on iodothyronine metabolism might also exist since serum rT3 increased 49 .+-. 19% in patients with treated myxedema. In normal subjects and diabetic patients only a trend towards such a variation was observed.