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 tamoxifen. Tamoxifen treatment suppressed plasma levels of FT3 and FT4 (p<0.005 for both) and elevated plasma concentrations of TBG (p<0.005) and TG (p<0.025). In general, these changes became significant after 6 months of treatment. Plasma TSH increased significantly after 1 y of treatment (p<0.025). A fall in FT4 and FT3 combined with increase in TSH suggests a reduced bioavailability of T4 and T3 during tamoxifen treatment. The increase in TG may reflect a reduced synthesis or liberation of T4 resulting in a reduced plasma level of FT4. Our findings suggest that tamoxifen influences the thyroid hormone levels, not only by modulating plasma TBG, but also by interfering with hormone synthesis or secretion in the thyroid gland.