Seasonal variations of glucose and triiodothyronine concentrations in serum of carp (Cyprinus carpio L).

Thyroid hormone and glucose serum concentrations (SC) of carps (Cyprinus carpio L) have been monitored at weekly intervals throughout the year. T4 and rT3 concentrations were always below the limit of detection of the assays applied. T3 and glucose mean SC showed seasonal variations. Highest T3 SC were reached in summer with a peak value in August and lowest values in winter with a maximum in December. Glucose SC have been found to be highest in winter and lowest in summer. Statistical analysis showed a negative correlation between T3 and serum glucose SC (r = -0.77, P less than 0.001). The seasonal dependency of T3 and glucose SC suggest that either temperature, food intake or day-light or both might play a role in the regulation of glucose metabolism as well as of T3 production or metabolism.