EFFECT OF ADMINISTRATION OF GROWTH-HORMONE ON PLASMA AND INTRACELLULAR LEVELS OF THYROXINE AND TRIIODOTHYRONINE IN THYROIDECTOMIZED THYROXINE-TREATED RATS

We have studied the effects of the administration of GH on plasma levels and peripheral production of tri-iodothyronine (T3) from thyroxine (T4) in thyroidectomized male Wistar rats given a continuous i.v. infusion of T4 (1-mu-g/100 g body weight per day) and GH (120-mu-g per day) for 3 weeks. Tracer doses of I-131-labelled T3 and I-125-labelled T4 were added to the infusion. At isotopic equilibrium (10 days after the addition of I-125-labelled T4) the rats were bled and perfused. The plasma appearance rate for T3 was higher (10.6 +/- 1.3 vs 8.4 +/- 2.8 pmol/h per 100 g body weight, P = 0.05) and plasma TSH was lower (246 +/- 24 vs 470 +/- 135 pmol/l, P < 0.01) in GH-treated rats. The amount of T3 in liver (12.3 +/- 2.8 vs 5.5 +/- 1.7 pmol/g wet weight, P < 0.01), kidney (11.5 +/- 1.4 vs 6.5 +/- 1.4 pmol/g wet weight, P < 0.01) and pituitary (8.8 +/- 2.7 vs 4.8 +/- 0.5 pmol/g wet weight, P < 0.01) was higher than in controls, mainly as a result of an increased local production of T3 from T4, but plasma-derived T3 was also higher in most organs. We found an increased intracellular T3 concentration in the pituitary which may be responsible for the lower plasma TSH concentration in the GH-treated rats. Since the increase in locally produced T3 is found particularly in liver, kidney and pituitary, typical organs that express 5'-deiodinase activity, we suggest that GH acts on thyroid hormone metabolism by stimulating type-I deiodinase activity.