Effects of diabetes, beta-hydroxybutyric acid and metabolic acidosis on the pituitary-thyroid axis in the rat.

Previous studies demonstrated alterations of thyroidal economy in untreated diabetes mellitus both in man and experimental animals. To test the role of beta-hydroxybutyric acid (BHB) and acidosis in generating such changes, we studied the pituitary-thyroid axis of streptozotocin-diabetic rats, BHB or ammonium chloride (NH4Cl)-treated normal rats. Serum TSH, pituitary content and pituitary concentration of TSH, serum T4, T3 and free T4 (FT4), were all measured by RIA. In short term (2 days) diabetic rats the pituitary content of TSH was normal whereas the concentration (per mg of protein) was elevated (p less than 0.05 versus control group). Serum TSH (p less than 0.05), serum T4 (p less than 0.05), serum T3 (p less than 0.01) and serum FT4 (p less than 0.05) were all significantly decreased. In long term (30 days) untreated diabetic rats serum changes were similar to the short term diabetic group, though the pituitary content of TSH was significantly decreased (p less than 0.05). Animals treated with NH4Cl had no variations from controls. However, rats treated with BHB displayed a significant decrease in pituitary content of TSH (p less than 0.05), pituitary concentration of TSH (p less than 0.05) and in plasma TSH (p less than 0.01), and normal thyroid hormones in serum. No significant changes were seen in the TSH response to TRH in 2 or 30 days untreated diabetic and in BHB - treated animals. The data suggest that BHB, although not NH4Cl acidosis, may be capable of inducing a moderate depression of pituitary and plasma TSH of a lesser magnitude of that accompanying the full, long term diabetic state in the rat.