Seasonal Variation in Thyroid Gland Activity in Pheasants

Abstract

Thyroid glands from 167 pheasants (Korean-Phasianus colchicus karpowi, Persian- Phasianus colchicus persicus, green-Phasianus versicolor, and Reeves-Syrmaticus reevesii) were examined to determine if there were a seasonal or sexual variation in (1) the weight of the thyroid gland and (2) thyroid activity as determined by histological techniques. A Thyroid Activity Index, which histologically rates thyroid activity from 1 to 5 (inactive to very active), was employed to evaluate the functional state of the gland. Five males and five females each of the pheasant groups were sacrificed in January, April, July, and October. Generally, the thyroid glands were most active in January and April and least active in July and October. An inverse relationship seemed to exist: the lighter the gland, the more active it is, and the heavier the gland, the less active it is.