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

• Published in: Canadian Journal of Zoology
• Main Authors: Boeuf, Gilles, Uin, Linda M., Eales, J. G.
• Format: Article in Journal/Newspaper
• Language: English
• Published: Canadian Science Publishing 1989
• Subjects: Animal Science and Zoology; Ecology, Evolution, Behavior and Systematics; Atlantic salmon; Salmo salar
• Online Access: http://dx.doi.org/10.1139/z89-237; http://www.nrcresearchpress.com/doi/pdf/10.1139/z89-237

To determine if the marked spring surges that occur in plasma L-thyroxine (T 4 ) concentrations during parr–smolt transformation are due to changes in the properties or levels of thyroid hormone binding plasma proteins, plasma levels of total T 4 (TT 4 ) and 3,5,3′-triiodo-L-thyronine (TT 3 ), indices of the percentage of T 4 or T 3 in the free form (%FT 4 or %FT 3 ), and indices of free T 4 (FT 4 ) and free T 3 (FT 3 ) 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 TT 4 exhibited two major peaks in late April and mid-June, with small seasonal change in TT 3 . Indices of the %FT 4 and %FT 3 changed negligibly, resulting in high correlations between TT 4 and FT 4 (r = 0.97) and TT 3 and FT 3 (r = 0.96). Thus the high TT 4 peaks encountered during smoltification are not due to changes in properties of plasma T 4 binding proteins and reflect approximately proportionate changes in the putative physiologically relevant FT 4 level. The TT 4 and FT 4 peaks coincided with basal levels of TT 3 and FT 3 , implying no systemic increase in T 4 to T 3 conversion and reinforcing the concept that T 4 may exert a local tissue-specific role during parr–smolt transformation.