Seasonal and diurnal rhythms of thyroidal status in the rainbow trout, <i>Salmo gairdneri</i> Richardson

Abstract

The concentrations of thyroxine (T 4 ) and triiodothyronine (T 3 ) in the blood plasma of rainbow trout ( Salmo gairdneri Richardson) at intervals throughout the year have been measured by a chemically specific gas‐liquid chromatographic (GLC) method. Mean hormone levels showed a seasonal variation, maximal levels of both hormones occurring in winter and minimal concentrations in mid summer. An apparent secondary maximum in mean T 4 and T 3 concentrations was observed in spring. (T 4 )/(T 3 ) ratios have been found to be highest in winter and lowest in summer. A radioimmunoassay procedure, validated by GLC analyses, revealed the presence of a diurnal rhythm in serum T 4 levels of trout sampled in September and in April.