Ovarian iodide uptake and triiodothyronine generation in follicular fluid - The enigma of the thyroid ovary interaction

Since 1928, the iodine concentration in the ovary has been known to be higher than in every other organs except the thyroid. The ovarian iodide uptake varies with sexual activities, is enhanced by estrogens and a hypothyroid state and blocked by goitrogens. The recent discovery of a sodium iodide symporter (NIS) in ovaries has offered a possible mechanism for ovarian iodide uptake and other functional similarities to its thyroid counterpart. Nevertheless, the physiological significance of ovarian iodine uptake and accumulation remains unknown. The presence of thyroid hormones (TH) in follicular fluid (FF) has been established recently. Our preliminary studies on TH in FF (1996-1998) in rabbits, pigs, horses showed that the concentration of T-4 is generally lower than that in serum and that for T-3 is within the normal range or higher. A positive correlation exists between the T-4 levels in FF and serum but not between the corresponding T-3 levels. These studies revealed, for the first time, the presence of the ovarian 5'-monodeiodinase system in FF capable of generating T-3 (ovary-born T-3) by outer ring deiodination of T-4. In mares, seasonal polyestrus, ovarian 5'-monodeiodinase (MD) activity and FF T-3 levels have been found to be higher during the ovulatory period than in the anovulatory one. The exact physiological significance of this system generating T-3 and coexisting with isoforms of TH receptors in granulosa cells has not been elucidated. A direct role of T-3 for the early follicular development, differentiation and for the steroidogenic capability of granulosa cells, although strongly suggested by data obtained from in vitro studies, has to be elucidated. (c) 2005 Elsevier Inc. All rights reserved.