NOTE Changes in Circulating L-Thyroxine and L-Triiodothyronine of the masu salmon, Oncorhynchus masou accompaning the smoltification, Measured by Radioimmunoassay

Abstract

Serum levels of L-thyroxine (T₄) and L-triiodothyronine (T₃) of the masu salmon, Oncorhynchus masou, were measured by radioimmunoassay during the months from March to September. Serum levels of T₄ and T₃varied from0.06to1.44μg and from0.06to0.35μg per100ml serum respectively, although T₄content could not be detected in some specimens.
T₄ level showed a seasonal fluctuation in which a higher value was observed in April when parr-smolt transformation was proceeding. T4 level in spring, particularly in April and June, was significantly higher in smolt than in parr.
T₃ content showed a small variation during the observed monthsand did not increase at smoltification, although the content tended to decrease to a minimum level in September when T₄ level showed also the lowest level.