Thyroid morphology and function and its role in thermoregulation in the newborn southern elephant seal (Mirounga leonina) at Macquarie Island.

The thyroid gland of the newborn southern elephant seal pup exhibits markedly increased secretory activity during the first 24 hours after birth. Thyroid epithelial cell height is cuboidal to columnar for pups from birth to 48 hours postpartum after which it decreases by five days of age. Ultrastructurally the thyroid epithelial cells show pseudopodia protruding into the lumen at zero, two and six hours after birth. After 24 hours postpartum pseudopodia are rarely observed in thyroid follicles from two to 20 days old pups. The number of colloid droplets increases by six hours after birth and they are distributed throughout the cytoplasm. At 24 hours and two days, few colloid droplets are observed. Plasma T4 concentration increases three-fold from birth, to peak at six hours postpartum after which it steadily declines. Plasma concentration of T3 increases eight-fold between birth and 24 hours postpartum. T3 levels remain high until five days to seven days, then decrease to 20 days. The observed changes in thyroid epithelial cell height and ultrastructure is strongly suggestive of increased secretion of thyroid hormones during the first six hours of postnatal life. This pattern of thyroid activity is similar to that in other newborn mammals which have been examined. The thyroid gland of the southern elephant seal is markedly active at birth and is responsible for the elevated levels of T4 and T3, thus playing a vital role in maintaining the body temperature of the newborn seal when it enters the harsh environment of the sub-Antarctic.