Chronic estradiol exposure modulates thyroid structure and decreases T4 and T3 serum levels in middle-aged female rats

OBJECTIVES: In human medicine, estrogen is applied in prevention and treatment of health problems associated with the menopause. The aim of this study was to examine the effects of chronic estradiol dipropionate (EDP) treatment on thyroid gland structure and function in middle-aged female rats. METHODS: At 14 months of age, Wistar rats received 0.625 mg EDP/kg b.w./day intraperitoneally for 2 weeks. The peripheral and central zones of the thyroid were stereologically analyzed and the following morphometric parameters determined: volume density of follicles, follicular epithelium, interstitium and colloid, epithelial height and the index of activation rate. Serum levels of TSH, T4 and T3 were determined by ELISA. RESULTS: EDP treatment led to significant decreases in volume densities of follicles and follicular epithelium, epithelial height and index of activation rate (by 11%, p < 0.05; 23%, p < 0.005; 11%, p < 0.05 and 21%, p < 0.05, respectively) in comparison to control values. Hyperplasia of thyroid follicular cells was noticed in 25% of EDP-treated animals. Serum levels of T4 and T3 were decreased (by 33%, p < 0.005 and 28%, p < 0.001, respectively), but TSH concentration was not significantly different from that of the controls. CONCLUSION: Chronic estradiol treatment significantly decreased volume density and height of centrally located follicular epithelium, follicular activation index and serum level of total thyroid hormones in middle-aged rats.