Thyroxine effects on serum insulin-like growth factor I levels, anthropometric measures, and body composition in patients after thyroidectomy.

Thyroxine is an important hormone related to growth. To study the effects of thyroxine on serum levels of insulin-like growth factor I (IGF-I), anthropometric measures, and body composition, we studied 28 patients who were receiving thyroxine therapy after thyroidectomy for well-differentiated thyroid cancer. Serum IGF-I levels decreased from 38.3 +/- 19.0 ng/ml (mean +/- SD) to 26.8 +/- 12.8 ng/ml (p <0.001) after withdrawal of thyroxine and increased to 43.8 +/- 20.2 ng/ml (p <0.001) after thyroxine was resumed. The serum levels of TSH, T3, and thyroxine changed accordingly after withdrawal and resumption of thyroxine therapy. Skinfold thickness increased from 21.8 +/- 6.5 mm to 23.7 +/- 6.4 mm (p <0.001) after withdrawal of thyroxine and decreased to 21.5 +/- 6.9 mm after resumption of thyroxine. Similar changes occurred in the circumferences of the waist and hip (p <0.01), but no significant change occurred in arm muscle circumference (18.7 +/- 2.3 cm vs 18.7 +/- 2.3 cm vs 19.0 +/- 2.4 cm). Body composition analysis by bioelectrical impedance showed no significant changes in the percentages of body fat and lean mass. In conclusion, withdrawal of thyroxine from patients with thyroid cancer was associated with an immediate decrease in serum IGF-I levels. However, lean mass did not respond to the changes in serum IGF-I levels.