Thyroid hormones in grey seal pups (Halichoerus grypus)

The aim of the present study was to describe the changes in thyroid hormone status in grey seal (Halichoerus grypus) pups from birth to weaning and mo ulting. Plasma concentrations of total thyroxine (tT(4)) were highest the f irst two days after birth, thereafter dropping to a lower, but stable level . This pattern may reflect a high transfer rate of maternal thyroxine prepa rtum, prior to parturition, or postpartum via colostrum, or it may be cause d by active secretory thyrocytes in late foetal stage. Total triiodothyroni ne (tT(3)) concentrations were lowest in neonatal pups, and increased as a function of age, indicating that deiodination of T-4 to T-3 increases as a function of age. Plasma concentrations of free thyroxine (fT(4)) did not va ry as a function of age. All hormone concentrations were higher than previo usly reported in adults, probably reflecting the important role of these ho rmones in regulating their high rates of metabolism and tissue synthesis an d the growth of their juvenile pelage. Since polychlorinated biphenyls (PCB s) have been reported to affect plasma concentrations of thyroid hormones i n seals, the information on thyroid hormone concentrations and dynamics rep orted in grey seal pups from a pristine Norwegian coastal environment provi de valuable reference material for future studies on pups from more pollute d areas. (C) 1999 Elsevier Science Inc. All rights reserved.