Abstract
The effect of thyroid hormone on the serum level of somatomedin A was studied using a radioreceptor assay for somatomedin A. In 13 patients with hyperthyroidism, the mean level of somatomedin A was 0.60±0.05U/ml, which was within the normal range. Low levels were found in 19 patients with primary hypothyroidism with a mean of 0.39±0.04 U/ml. After treating these patients with thyroxine, the serum somatomedin A levels returned to the normal range. There was a significant correlation between the somatomedin A level on one hand and T3 (r=0.49) and T4 (r=0.55) level on the other.
The serum level of somatosmedin A did not decrease for 7 days after thyroidectomy nor increase after treating hypophysectomized rats with 2 μg of thyroxine for 14 days. However, the effect of growth hormone on serum somatomedin A was augmented 42.2% by the combined administration of the same amount of T4 for 7 days. There was a positive correlation between the serum level of somatomedin A and the percentage increase in body weight (r=0.73). These data suggest that thyroxine potentiates the effect of GH on synthesis and/or the secretion of somatome din A.
