THYROID-FUNCTION IN POSTMENOPAUSAL BREAST-CANCER PATIENTS TREATED WITH TAMOXIFEN

Thyroid function was evaluated in 26 postmenopausal women with breast cancer before and at various time intervals during treatment with tamo xifen. Tamoxifen treatment suppressed plasma levels of FT3 and FT4 (p< 0.005 for both) and elevated plasma concentrations of TBG (p<0.005) an d TG (p<0.025). In general, these changes became significant after 6 m onths of treatment. Plasma TSH increased significantly after 1 y of tr eatment (p<0.025). A fall in FT4 and FT3 combined with increase in TSH suggests a reduced bioavailability of T4 and T3 during tamoxifen trea tment. The increase in TG may reflect a reduced synthesis or liberatio n of T4 resulting in a reduced plasma level of FT4. Our findings sugge st that tamoxifen influences the thyroid hormone levels, not only by m odulating plasma TBG, but also by interfering with hormone synthesis o r secretion in the thyroid gland.