Role of thyroid hormones in tilapia larvae (Oreochromis mossambicus): II. Changes in the hormones and 5'-monodeiodinase activity during development.

Thyroid hormone profiles and 5'-monodeiodinase activity were determined in tilapia at different stages of early development. The results showed that both T4 and T3 were present in significant amounts in fertilized eggs. There was a steady decrease in both T4 and T3 levels during embryonic development. The levels continued to decline after hatching until around 7 days later when most of the yolk had been absorbed. The T4 level started to rise then, suggesting that the larval thyroid had begun to produce T4 at this time, which coincided with the period of faster growth of the larvae. The T3 level remained fairly constant until around 20 days after which it rose significantly. In vitro determination of 5'-monodeiodinase activity (5'-D activity) in the whole-body homogenates of larvae showed that the enzymatic conversion of T4 to T3 was not detectable in eggs and 3-day-old larvae but detected in 5-day-old and older larvae. There was a gradual increase in the Vmax as development proceeded indicating increasing 5'-D activity during larval development. The Km values did not differ significantly in the different stages of development. These results are discussed in relation to the growth and development of the larvae.