Pituitary thyrotroph function and thyroid hormones in longstanding type-II diabetes mellitus before and after insulin treatment.

We investigated the TRH induced TSH secretion and thyroid hormone concentrations in 14 type-II diabetics with secondary failure to oral hypoglycemic agents (mean age: 70.8 years, range: 55-77, mean duration of disease: 11 years +/- 7.9 SD, mean HbAlc: 10.1 +/- 1.6 SD) with normal body weight before and after insulin therapy. Results were compared with those of healthy control persons. Before insulin treatment T3 and fT3 concentrations were significantly lower in diabetics (p less than 0.05) compared to controls, rT3 was slightly elevated in diabetics. With improved metabolic control following insulin treatment T3 and fT3 slightly increased, whereas rT3 slightly decreased. Basal TSH concentrations and TRH induced TSH secretion were not altered in diabetics compared to controls and were not influenced by insulin therapy. The alterations of peripheral thyroid hormone concentrations seem to be caused by reduced peripheral deiodination of T4 to T3 with increased production of rT3. Pituitary thyrotroph function is normal in long-standing type-II diabetic patients with normal body weight.