Effects of experimentally induced mild hyperthyroidism on growth hormone and insulin secretion and sex steroid levels in healthy young men

Although triiodothyronine (T₃) exerts major regulatory actions in both animals and humans, most clinical studies of T₃ administration have been relatively short-term. The present study examined the effects of more than 2 months (63 days) of low-dose T₃ treatment on overnight pulsatile growth hormone (GH) secretion, short-term insulin secretion, and of sex steroid levels in seven healthy, lean men studied at an inpatient metabolic unit. At baseline, there were strong correlations between sex hormone-binding globulin (SHBG) and several measures of GH production, including total GH production (r = .99), GH interburst interval (r =-.75), and GH mass (r = .82). SHBG was also inversely correlated with basal insulin secretion (r = -.74). There was a 42% increase in serum levels of total testosterone (18.5 ± 1.3 to 26.3 ± 1.8 nmol/L, P = .005) and a 150% increase in SHBG (18.0 ± 2.2 to 44.9 ± 7.0 nmol/L, P = .008) following T₃ treatment. Estradiol and free testosterone levels were unchanged by treatment, although free testosterone decreased from 142.8 ± 18.4 to 137.3 ± 19.5 pmol/L. T₃ treatment significantly reduced the GH interburst interval (P < .05) and produced slight increases in the measures of GH secretion. There were no statistically significant effects of T₃ treatment on insulin secretion, although insulin peak amplitude, mass secreted per burst, and total production all decreased. We conclude that experimentally induced T₃ excess in healthy men produces significant and sustained changes in sex hormone levels and GH secretion. Furthermore, there are strong associations between SHBG and both GH and insulin secretion independent of thyroid hormone excess that require additional study.