Thyroid hormone concentrations in systemic circulation and ovarian follicular fluid of cows

Abstract

The aim of this study was to determine and compare the concentrations of total (T) and free (F) fractions of thyroid hormones (T₃-triiodithyronine and T₄-thyroxin) in peripheral circulation and follicular fluid of cows in relation to ovarian follicular status in vivo (Experiment 1), and in the follicles from the slaughterhouse ovaries (Experiment 2). In Experiment 1, estrus was synchronized in 15 cows using two Estrumate^® (cloprostenol sodium) injections (250 mg cloprostenol intramuscular), the time of ovulation (Day 0) was confirmed by ultrasonography, and ovarian antral follicles were ablated on Day 5. The ensuing superovulatory treatment consisted of eight Folltropin^®-V injections (50 mg intramuscular) administered twice daily from Day 6 to Day 9, followed by two injections of Estrumate^® (Day 10 am and pm) and a single dose of Lutropin Alfa^® (Day 11; 750 IU intramuscular). On Day 5, both TT₃ and FT₃ concentrations were greater (P < 0.05) in serum than follicular fluid from dominant (DFs) or subordinate antral follicles (SFs), and TT₄ concentrations were greater (P < 0.05) in DFs compared with SFs. Serum concentrations of FT₄ were greater (P < 0.05) on Day 12 than on Day 5, and TT₄ concentrations in follicular fluid collected on Day 12 were higher than those in DFs and SFs on Day 5. In Experiment 2, there were no differences (P > 0.05) in thyroid hormone concentrations between the largest and all remaining antral follicles visible on the surface of the ovary (n = 20 ovaries). We concluded that: (i) physiological status of bovine antral follicles (i.e. dominant versus subordinate) may impinge on the accumulation of TT₄ in follicular fluid; and (ii) hormonal ovarian superstimulation increases circulating levels of FT₄ and follicular fluid content of TT₄.