Postnatal changes in concentrations of serum and urinary thyroxine and 3,3',5-triiodothyronine in the pig

Urine and serum obtained from piglets at birth (0 hr), 6, 12, 24, 48, 72 and 240 hr postpartum were assayed for thyroxine (T4) and 3, 3', 5-triiodothyronine (T3) concentrations. Urine was also quantified for total iodine. Piglet body weights and hematocrits were also recorded at each sampling time. Serum T4 concentrations were higher (P < .05) at birth (0 hr) than at any other time. By 24 hr, serum T4 decreased (P < .05) to a concentration similar to that observed at 240 hours. In contrast, serum T3 concentrations were lowest (P < .01) at birth but doubled (P < .01) within 6 hours. At 12 and 24 hr, T3 concentrations were lower (P < .05) than they were at 6 hours. By 48 and 72 hr, serum T3 concentrations increased to values similar (P < .05) to those observed at 6 hr; they then decreased (P < .05) slightly by 240 hours. Urinary concentrations of T4 and T3 were lowest (P < .05) at birth and increased (P < .05) by 12 hours. Urinary T4 concentration at 72 hr decreased (P < .05) to values observed at birth (0 hr), while T3 concentrations at 72 hr remained similar to values observed at 12 hours. T4 and T3 concentrations at 72 hr remained similar to values observed at 12 hours. T4 and T3 iodine accounted for only .16 +/- .025% of the total iodine in the urine. Hematocrits (PCV percentage) decreased (P < .01) during the first 24 hour of the pigs's life. High (P < .005) linear relationships existed among serum T3, body weights and serum T4.