Seasonal Changes in Serum Total Thyroxine, Free Thyroxine, and Canine Thyroid-Stimulating Hormone in Clinically Healthy Beagles in Hokkaido

Abstract

The purpose of this study was to evaluate seasonal influences on thyroid hormone levels of healthy outdoor dogs in Hokkaido. We surveyed serum basal total thyroxine (tT₄), free thyroxine (fT₄), and canine thyroid-stimulating hormone (cTSH) levels, and tT₄ levels after administration of TSH for a year. Basal tT₄ levels decreased in January, and increased in August and September. fT₄ levels increased in January and November. No significant seasonal variation was found in cTSH. tT₄ levels after administration of TSH in August and November increased. These results suggested that the thyroid gland may have been activated in November. We should take seasonal variation into consideration when thyroid function is tested.