The effect of tamoxifen therapy on thyroid function tests

Serum tri-iodothyronine (T3), thyroxine (T4) and thyroid-stimulating hormone (TSH) concentrations were measured in 50 postmenopausal patients with breast cancer receiving tamoxifen therapy. Ten patients (20%) showed an elevation of serum total T4 concentrations and two of these patients showed additional elevation of serum total T3 levels. One patient had a persistently raised free T4 concentration and was considered to be thyrotoxic. Serum thyroxine-binding globulin (TBG) was estimated in six patients with elevated total T4 concentrations and was found to be raised in each case. Total serum T4 concentrations rose in seven of ten additional patients after starting tamoxifen therapy. However this change did not reach statistical significance. This trend in serum T4 concentrations was not shown in ten normal volunteers taking tamoxifen for 1 week.