Annual variations in the circulating levels of thyroid hormones in the brook trout, Salvelinus fontinalis, as measured by radioimmunoassay

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Abstract
Circulating levels of L-thyroxine (T4) and 3,5,3′-triiodo-L-thyronine (T3) in plasma of adult brook trout, Salvelinus fontinalis (Mitchill), were measured by radioimmunoassay. Plasma levels of T4 varied from 0.8 to 3.7 ng/ml and T3 from 2.4 to 8.6 ng/ml over a 12-month period. Maximal levels of both hormones occurred in midspring and minimal levels in early November at the time of spawning. T3 titres were consistently higher than those of T4. All plasma samples were collected between 1030 and 1200 hours. A pilot study suggests that T3 and T4 concentrations also vary on a diurnal basis.