Serum thyroid hormone levels during growth period in labrador dog

72 Labrador dogs were divided into six groups according to their ages with 12 animals with equal no. of male and females in each group. The circulating level of T₃ and T₄ was measured by radio immuno assay (RIA) Serum T₃ significantly differ during growth period in dogs, while serum T₄ concentration did not differ significantly. Concentration of mean serum T₃ was lower in the younger dogs and it increases with advancing age. The T₃ values recorded at 3 – 6 months of their age were significantly higher than the values recorded at 0 -3 month of their age. Sex did not affect serum T₃ concentration in dogs. The opposite relationship viewed between age and serum T₄ concentration during the growth period. The serum T₄ levels declined during 4–6 months of their age. Sex had no statistically significant effect on the mean serum T₄ concentration. Serum T₄:T₃ ratio differed significantly in various age groups in growth period and ratio showed decreasing trend as age advances.