Changes in tissue and blood concentrations of thyroid hormones in developing chum salmon

The changes in tissue and blood concentrations of thyroxine (T4) and triiodothyronine (T3) were examined during development of the chum salmon (Oncorhynchus keta). Extraction methods previously established for tissue T4 were also validated for tissue T3, by parallel displacement curves to T3 standard in the radioimmunoassay and by the same elution patterns of immunoreactivity in a HPLC system. The T3 concentration of the eggs just after fertilization (4-9 ng/g) was lower than the T4 concentration (5-15 ng/g). Both T4 and T3 concentrations in the whole body decreased steadily during yolk absorption, primarily due to the decline of the hormone content in the yolk. Both T4 and T3 were detected in blood plasma at later stages of yolk absorption, and the plasma levels increased toward the end of yolk absorption. At the end of yolk absorption, when the larvae emerge from the gravel bed, a transient increase in whole body concentrations of T4 and T3 was observed. Plasma levels of T4 were always greater than the T3 levels. Thyroid follicles began to develop during the early stages of yolk absorption. These findings suggest important roles of maternal thyroid hormones for developing salmon embryos during yolk absorption.