The effect of droloxifene and estrogen on thyroid function in postmenopausal women

Estrogen is known to increase serum T4-binding globulin (TBG) concentrations, thereby increasing serum total T4 concentrations. Serum free T4 concentrations, however, remain normal. Tamoxifen, a selective estrogen receptor modifier (SERM), also raises serum TBG concentrations, but whether newer SERMs with less stimulatory action on the endometrium do so is not known. We, therefore, compared the effect of droloxifene, a SERM, and conjugated equine estrogen on pituitary-thyroid function in normal postmenopausal women. Ten women were treated for 6 weeks with conjugated estrogen (Premarin), 0.625 mg/day, and droloxifene, 60 mg/day, in a double-blind crossover study with an intervening 4-week no-treatment period. We measured serum T4, T3, TBG, free T4 index, and TSH at baseline and at the end of each 6-week period. The baseline values were compared with the 6-week values using paired t tests. The mean (+/- SD) serum TBG concentrations increased significantly during both treatment periods (baseline, 1.5+/-0.4 mg/dL; conjugated estrogens, 2.7+/-0.6 mg/dL; droloxifene, 2.1+/-0.6 mg/dL; P < 0.001 and P = 0.001, respectively). There were no significant changes in the serum free T4 index. Serum T4 and T3 concentrations increased during both treatment periods, however, the increase was significant only for T4 during the conjugated estrogen treatment period. The serum TSH concentrations increased significantly during both treatment periods (18% during conjugated estrogen and 11% during droloxifene), and the values remained within the normal range in all women. Administration of both conjugated estrogen and droloxifene for 6 weeks increases serum TSH and TBG concentrations, but does not alter free T4 index values in postmenopausal women.