Gender-related differences of serum thyroxine-binding proteins in the rat

Because little information is available, studies were performed to determine the relationship between gender and sex steroid status on serum T4 binding proteins in the rat. The binding capacity of serum thyroxine-binding globulin was greater in female rats than in male rats (27 +/- 1.3 vs 18.0 +/- 1.3 nmol/l, p less than 0.01) and in fasted female rats than in fasted male rats (64.4 +/- 2.6 vs 30.8 +/- 2.7 nmol/l, p less than 0.01). The binding capacity of serum transthyretin was lower in female rats than in male rats (2.1 +/- 0.1 vs 3.1 +/- 0.1 mumol/l, p less than 0.01). Neither ovariectomy or orchidectomy affected the binding capacity of serum thyroxine-binding globulin and it was not increased in ovariectomized rats treated with estrogen. Orchidectomy did not cause a decrease in the binding capacity of serum transthyretin and testosterone administration did not increase it. In contrast, ovariectomy caused an increase in the binding capacity of serum transthyretin (Intact = 2.2 +/- 0.1 vs ovariectomized = 2.8 +/- 0.1 mumol/l, p less than 0.01) and estrogen administration caused a decrease (ovariectomized = 2.8 +/- 0.1 vs ovariectomized + E2 = 1.9 +/- 0.1 mumol/l, p less than 0.05). The results indicate that the binding capacity of serum thyroxine-binding globulin is higher in female rats than in male rats but this difference is not due to differences in the secretion of gonadal hormones. The binding capacity of transthyretin is lower in female rats than in male rats. This is probably due to the higher circulating levels of estrogen in the female compared to the male.