Seasonal Changes of Morphometric Structure and Plasma Hormone Levels of Thyroid Gland in Persian Gulf Yellowfin Seabream (Acanthopagrus latus)

Seasonal changes of the thyroid gland structure and hormones secretion was examined in yellowfin seabream Acanthopagrus latus in Persian Gulf (Zangi branch, Musa creek). Thyroid gland composed of follicles scattered around the ventral aorta, near the gills. Follicular cells varied according to secretion of the gland during warm and cold seasons. Thyroid hormones (Triidothyronine [T3] and Thyroxine [T4]) were detected in the fish serum in levels ranged from 4.09-1.3 ng/mL for T3 and 1.1-0.21 ng/mL for (T4) in the warm and cold seasons, respectively. The results showed that the height of thyroid epithelium and plasma concentration of thyroid hormones (thyroid activity) in Acanthopagrus latus increased significantly during spring and summer. The peak of these factors occurred in midsummer (August). Then, the thyroid activity decreased significantly during autumn and early winter from October to December according to decreasing of temperature. T3 and T4 increased significantly from January to April.