Patterns of thyroxine and triiodothyronine in serum and follicle-bound oocytes of the tilapia, Oreochromis mossambicus, during oogenesis.

This study describes simultaneous measurements of thyroid hormones, thyroxine (T4) and triiodothyronine (T3), in the oocytes and serum of a female teleost fish over a complete reproductive cycle. We have identified patterns in circulating T4 and T3 levels as well as their accumulation into oocytes during the reproductive cycle of the tilapia (Oreochromis mossambicus). This is the first description of the patterns with which thyroid hormones accumulate in teleost oocytes. The sampling strategy used in the study eliminated the possible influences of covarying environmental factors that may affect thyroid hormone levels independently of reproductive events. Hormones in serum and oocytes were measured by radioimmunoassay utilizing miniature Sephadex columns. The total content of both thyroid hormones in the oocytes increased throughout most of the ovarian cycle as the oocytes increased in size from less than 2 mg to approximately 6.5 mg by ovulation. By contrast, concentrations of thyroid hormones in the oocytes rose only during the first third of post-spawning oocyte growth (up to approximately 2 mg) before attaining plateaus at approximately 6 ng/g for T4 and 13 ng/g for T3. Serum concentrations of T4 and T3 varied in cyclical patterns during oogenesis, dropping to lows of 3.4 ng/ml (T4) and 2.7 ng/ml (T3) when the oocytes were 1.5 and 2 mg, respectively, and then increasing to 6.5 ng/ml (T4) and 4.8 ng/ml (T3) when the oocytes reach approximately 6 mg. The concentrations of both hormones decreased shortly before spawning. Maximum concentrations of thyroid hormones in the oocytes were reached approximately 10 days prior to those in the serum. Although the serum levels of T4 were greater than those of T3, the reverse was found in the oocytes. Triiodothyronine appears to be accumulated selectively over T4 and the patterns with which both thyroid hormones accumulate in the oocytes of the tilapia do not appear to be tied to serum levels.