Plasma levels of free and bound thyroid hormones during parr–smolt transformation in Atlantic salmon, Salmo salar L.

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Abstract
To determine if the marked spring surges that occur in plasma L-thyroxine (T4) concentrations during parr–smolt transformation are due to changes in the properties or levels of thyroid hormone binding plasma proteins, plasma levels of total T4 (TT4) and 3,5,3′-triiodo-L-thyronine (TT3), indices of the percentage of T4 or T3 in the free form (%FT4 or %FT3), and indices of free T4 (FT4) and free T3 (FT3) levels were measured on a population of Atlantic salmon undergoing parr–smolt transformation under a natural temperature and photoperiod regime in Brittany, France. Gill Na+–K+ ATPase increased to a peak in mid-May, whereas TT4 exhibited two major peaks in late April and mid-June, with small seasonal change in TT3. Indices of the %FT4 and %FT3 changed negligibly, resulting in high correlations between TT4 and FT4 (r = 0.97) and TT3 and FT3 (r = 0.96). Thus the high TT4 peaks encountered during smoltification are not due to changes in properties of plasma T4 binding proteins and reflect approximately proportionate changes in the putative physiologically relevant FT4 level. The TT4 and FT4 peaks coincided with basal levels of TT3 and FT3, implying no systemic increase in T4 to T3 conversion and reinforcing the concept that T4 may exert a local tissue-specific role during parr–smolt transformation.

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