EFFECT OF THYROXINE AND GROWTH HORMONE ON LONGITUDINAL BONE GROWTH IN THE HYPOPHYSECTOMIZED RAT

Abstract

The effect of L-thyroxine and bovine growth hormone (NIH-GH-B16) on the growth in length from the proximal growth plate of the tibia in hypophysectomized female Sprague-Dawley rats was studied by the tetracycline method. The width of the growth plate was also determined, and the weight of the body and heart was registered. Completeness of the hypophysectomy was determined microscopically. Daily sc injections of 5, 10, 20 or 40 μg/kg L-thyroxine alone, or in combination with 25 or 100 μg NIH-GH-B16, were given for 20 days, starting 15 days after hypophysectomy which was performed when the rats were 60 days of age. Thyroxine alone resulted in stimulation of the longitudinal bone growth with an optimum effect at 10–20 μg/kg. Further increase of the thyroxine dose did not increase the bone growth. Thyroxine given in association with growth hormone had a higher growing promoting effect than thyroxine or growth hormone alone. The growth stimulation of the two hormones was also significantly higher than the expected additive effect, indicating a potentiating synergism. When thyroxine and growth hormone were given in combination, the longitudinal bone growth reached an optimum for almost the same dose (20 μg/kg) of thyroxine as when it was given alone. At this optimum dose of thyroxine, the dose of growth hormone determined the longitudinal bone growth. The width of the growth plate was not influenced by thyroxine administration. The body weight decreased somewhat when thyroxine was given alone, and the combination with growth hormone seemed to compensate for this weight loss. The heart weight was found to increase with increasing doses of thyroxine both when given alone and in association with growth hormone.