The influence of growth hormone and thyroxine on iodothyronine deiodinase activity in the liver, kidney and brown adipose tissue in hypophysectomized rats.

The effects of GH and T4 substitution on peripheral iodothyronine deiodinase activity in the liver, kidney and brown adipose tissue of hypophysectomized rats were investigated. Animals were treated with GH (140 micrograms hGH/day), T4 (3 micrograms/day), GH plus T4 (same doses), or saline. Rats were killed 0, 4, 7 or 11 days after treatment was started. Non-hypophysectomized, age-matched rats were killed after 0 and 11 days and served as controls. GH plus T4 restored body weight gain to normal, whereas GH alone and T4 alone did not. Tissue deiodinase activity and T3 concentrations were severely depressed in the hypophysectomized rats compared with non-hypophysectomized controls (to less than 10%). GH substitution in hypophysectomized rats led to a slight but significant elevation in tissue iodothyronine deiodinase activity in the liver and kidney, without concomitant increases in T3. Deiodinase activity in brown adipose tissue did not differ from that in saline-treated controls. T4 administration normalized deiodinase activity and tissue T3 content in all the evaluated tissues. GH plus T4 resulted in a lesser increase in deiodinase activity than T4 alone in the liver and kidney (p less than 0.01 at day 11), whereas no significant difference was observed in brown adipose tissue. In conclusion, GH stimulates iodothyronine deiodinase activity of the liver and kidney in hypophysectomized rats. Moreover, when GH is administered together with T4, the T4-stimulated enzyme activity in the liver and kidney is downregulated, suggesting that GH attenuates (or modulates) the T4 effect on this specific enzyme activity.