Circannual cycles of thyroid hormones in plasma of winter flounder (Pseudopleuronectes americanus Walbaum)

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Abstract
Seasonal changes in plasma concentrations of L-thyroxine (T4) and 3,5,3′-triiodo-L-thyronine (T3) were measured by radioimmunoassay for flounder bled shortly after capture (field fish) or after 7 days retention in the laboratory at seasonal temperatures and photoperiods (laboratory fish).In field fish plasma T4 was highest from April to June and lowest from November to February, whereas plasma T3 was highest from September to January and lowest in May to June. T4/T3 molar ratios increased markedly from March to June and fell to very low values during July to early February.Laboratory fish showed generally similar seasonal patterns, except that for most of the year their T4 levels were consistently higher and T3 levels consistently lower than their field counterparts, resulting in much higher T4/T3 ratios in plasma of laboratory fish.The above seasonal trends are discussed in relation to environmental and physiological parameters and stress.