The Effect ofd- andl-Thyroxine on Sex Hormone Binding Globulin in Rabbits*

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Abstract
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 wks) and restudied 6 wk 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 wk and values remained constant thereafter. Ovariectomy did not affect this age-related decrease in serum SHBG concentration. The 20- to 22 wk-old ovariectomized and sham-operated rabbits were treated daily with either 30 .mu.g/kg L-T4 or 150 .mu.g/kg D-T4 for 2 wk. This dose of L-T4 induced a 10-25% loss of BW [body weight] 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 wk after thyroid hormone therapy was discontinued. The 27- to 29-wk-old rabbits were then treated for 2 wk with D-T4 (150 .mu.g 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 < 0.01). D-T4 administration significantly lowered the serum cholesterol concentration without altering BW.