LOW CIRCULATING IGF-I LEVELS IN HYPERTHYROIDISM ARE ASSOCIATED WITH DECREASED GH RESPONSE TO GH-RELEASING HORMONE

OBJECTIVE Several abnormalities in the GH response to pharmacological stimuli have been described in hyperthyroidism. Both normal and high s erum IGF-l levels have been reported, as well as a decrease in IGF-I b ioactivity. We have evaluated the GH response to OH-releasing hormone (GHRH) in hyperthyroid patients and the effects of hyperthyroidism on serum IGF-l levels. The possible relations between nutritional status, thyroid hormones and IGF-I levels were also investigated. We also stu died the influence of long-term beta-adrenoceptor blockade on the OH r esponse to GHRH in these patients. DESIGN In 18 hyperthyroid patients and in 12 control subjects, GHRH (100 mu g) was administered as an i.v . bolus injection. Eight hyperthyroid patients and 8 control subjects received 50 mu g GHRH i.v. Seven hyperthyroid patients were reevaluate d after P-adrenoceptor blockade. IGF-l and albumin levels were measure d initially in all hyperthyroid patients and control subjects. Body co mposition was determined in 11 hyperthyroid patients and in a group of 33 matched normal controls. PATIENTS Hyperthyroid patients were compa red to control subjects. MEASUREMENTS GH, TSH and free T4 were measure d by immunofluorometric assay. IGF-l, total T3 and total T4 were measu red by radioimmunoassay. Body composition was determined using a dual- energy X-ray absorptiometer. RESULTS The GH response to 100 mu g GHRH in hyperthyroid patients was blunted compared to control subjects. The mean peak OH levels and the area under the curve were significantly l ower in hyperthyroid patients compared to control subjects (11 +/- 1 v s 27 +/- 5 mu g/l and 820 +/- 113 vs 1879 +/- 355 mu g/l 120min, respe ctively; P < 0.01). IGF-I levels were significantly reduced in hyperth yroid patients compared to controls (131 +/- 10 vs 201 +/- 16 mu g/l, respectively; P < 0.01). Ideal body weight, serum albumin levels and t he lean body mass were also reduced in hyperthyroid patients. After be ta-adrenoceptor blockade there were no changes in the blunted GH respo nse to GHRH in hyperthyroid patients. CONCLUSION Our data suggest that the blunted on response to GHRH in hyperthyroidism is apparently not related to circulating IGF-I levels. It is possible that nutritional f actors could play a role in the reduced circulating IGF-l levels found in these patients.