The Effect of d - and l -Thyroxine on Sex Hormone Binding Globulin in Rabbits*

Serum sex hormone-binding globulin (SHBG) concentration is increased in patients with thyrotoxicosis. SHBG is also present in rabbit serum, although it does not bind estradiol-17 beta (E2). Studies were carried out in female rabbits to determine the effects of thyroid hormone on SHBG. Serum concentrations of L-T4 cholesterol, E2, progesterone, free and total testosterone (T), and SHBG were measured in immature female rabbits (8-10 weeks of age). Rabbits were ovariectomized or subjected to sham surgery at puberty (age, 14-16 weeks) and restudied 6 weeks later. Values for serum T4, T, percent free T, free T, E2, progesterone and cholesterol were similar in ovariectomized and sham treated rabbits. Serum SHBG concentration progressively decreased in all rabbits from immaturity to age 20-22 weeks and values remained constant thereafter. Ovariectomy did not affect this age-related decrease in serum SHBG concentration. The 20- to 22-week-old ovariectomized and sham-operated rabbits were treated daily with either 30 micrograms/kg L-T4 or 150 micrograms/kg D-T4 for 2 weeks. This dose of L-T4 induced a 10-25% loss of BW, whereas D-T4 treatment did not, strongly suggesting that the L-T4 but not the D-T4-treated rabbits were hypermetabolic. D-T4 and L-T4 induced similar increases in serum SHBG (D-T4, delta 132 nM; L-T4, delta 146 nM). The increase in serum SHBG activity in response to D-T4 or L-T4 was reversible, since serum SHBG concentration returned to pretreatment values 5 weeks after thyroid hormone therapy was discontinued. The 27- to 29-week-old rabbits were then treated for 2 weeks with D-T4 (150 micrograms kg-1 day-1). Serum SHBG concentration significantly increased, and there were negative correlations between the thyroid hormone-induced increase in SHBG activity and both the percent free T and free T (P less than 0.01). D-T4 administration significantly lowered the serum cholesterol concentration without altering BW.