Blunted Growth Hormone (GH) Response to GH-Releasing Hormone in Hypothyroidism Resolves in the Euthyroid State*

Abstract

GH secretion is dependent upon thyroid hormone availability. In this study, the GH response to GH-releasing hormone (GHRH) was studied in a group of patients when they were hypothyroid and also when they were euthyroid. Hypothyroidism was associated with a significant reduction in both the peak GH response and the integrated GH secretory response to GHRH compared to those in the euthyroid state [4.7 ± 1.6 (±SEM) us. 12.2 ± 3.9 ng/ml (P < 0.025), and 349 ± 116 ng vs. 986 ± 304 ng ml^-1 min^-1 (P < 0.025), respectively]. GH responsiveness was impaired within 2 weeks of discontinuation of T₃ treatment in athyreotic subjects and was restored within 4 weeks of T₄ treatment in one chronically hypothyroid subject. The results imply that a blunted GH response to GHRH in hypothyroidism is attributable to a primary pituitary defect that occurs rapidly and is reversible with attainment of the euthyroid state.