Growth Hormone Secretion and Plasma Somatomedin‐c in Primary Hypothyroidism

Abstract

The effect of thyroid hormone deficiency on plasma immunoreactive somatomedin-C concentrations, growth hormone (GH) secretion in response to provocative stimuli, and the plasma somatomedin-C response to exogenous GH was studied in patients with primary hypothyroidism. Plasma Somatomedin-C concentrations were below the 95% confidence interval in 11 of 12 hypothyroid patients (mean +/- SD = 0.27 +/- 0.14 U/ml). With thyroid hormone therapy the mean plasma somatomedin-C level increased four-fold (1.00 +/- 0.43 U/ml). The capacity to secrete GH in response to pharmacological agents was impaired in 3 of the 6 hypothyroid patients tested and normal in the remainder. When the same 6 patients were given a single intramuscular injection of GH (0.1 U/kg) plasma somatomedin-C concentrations increased four-fold by 28 h after the injection. The magnitude of the somatomedin-C response was equal to or greater than that reported for euthyroid GH deficient subjects treated similarly. This study shows that plasma somatomedin-C concentrations are diminished by hypothyroidism. The decreased somatomedin-C levels do not appear to result from resistance to the stimulatory effect of GH, but may be either a result of diminished GH secretion or may be due to direct effects of hypothyroidism upon somatomedin production.

Effects of thyroid status on insulin-like growth factor-I, growth hormone and insulin are modified by food intake

“…All samples from littermate animals were analysed in the same batch. The intra-assay coefficients of variation for T 4 and T 3 measurements were less than 3.8% and less than 8.9% respectively.…”

“…Effects of thyroid status on IGF-I are modified by food 97 4 at a dose of 20 mg/kg body weight per day. Food intakes were either 30 or 60 g feed/kg body weight per day (3% or 6% respectively).…”

“…One of these animals was maintained hypothyroid, while the other two received thyroxine (T 4 ; Sigma), the dose of which was gradually increased to 20 mg/kg body weight per day by 1 week after thyroidectomy. This dose of T 4 was selected to give supraphysiological circulating levels of T 4 without rendering the animal grossly hyperthyroid, and was chosen according to a previously estimated distribution volume of approximately 2 l (14) and a half-life of nearly 1 day for T 4 (21) in animals of the same breed and similar age. For one of these hyperthyroid animals, the food given was increased to 60 g/kg body weight per day (6%) after thyroidectomy.…”

“…For one of these hyperthyroid animals, the food given was increased to 60 g/kg body weight per day (6%) after thyroidectomy. The doses of methimazole and T 4 were given orally with the feed. The fourth animal in each litter was kept, without any surgical manipulation, as a euthyroid control on 3% feed.…”

“…Insulinlike growth factor-I (IGF-I), one of the key mediators of cellular metabolism and proliferation, is directly influenced by growth hormone (GH) (1,2). Thyroid hormones (THs) can also modify plasma IGF-I levels, and a direct correlation between circulating levels of IGF-I and THs has been reported (3)(4)(5). The effects of THs on IGF-I may be mediated by stimulation of GH secretion, as has been demonstrated in pituitary tumour cell lines in vitro (6,7) and normal thyrotrophs in vivo (8,9).…”