Free thyroid hormones and cholesterol in follicular fluid of bovine ovaries

The aim of this study was to determine concentrations of free thyroxine (FT4), free triiodothyronine (FT3), cholesterol, and high density lipoproteins (HDL) in the bovine follicular fluid in relation to ovarian follicles size and season. The follicular fluid was collected in the spring and autumn from small (> 6 mm), medium (6 to 10 mm), and large (> 10 mm) follicles. The concentrations of FT4 in small follicles were higher (P < 0.05) than in large follicles. No significant differences were detected in levels of FT3, cholesterol, and HDL between the groups of follicles of different size. However, the intra-follicular concentrations of FT3 were higher than of FT4. Seasonal differences were observed in the concentrations of FT3 and cholesterol. In autumn the levels of FT3 were higher (P < 0.001) than in spring. The concentrations of FT4 were negatively correlated with the concentrations of cholesterol (P < 0.001) and HDL (P < 0.001). Negative correlations were found also between the concentrations of FT3 and cholesterol (P < 0.01). Our results confirm that thyroid hormones are required for bovine follicular function and indicate that the intra-follicular concentrations of FT3 and cholesterol may be affected by the season. Our findings suggest that thyroid hormones may be involved in cholesterol metabolism in bovine follicles.