The relationship of thyroxine secretion rate to growth of swine

The thyroxine secretion rate of five barrows and five gilts (Yorkshire x Landrace) was monitored at 4-week intervals from 10 to 26 weeks of age. Thyroxine secretion rate was determined by injecting 131I-thyroxine (20 MUCi) and monitoring both thyroxine concentration in plasma and the decline of labeled thyroxine in plasma over the next 48 to 60 hours. The thyroxine secretion rate increased from 10 to 14 and 14 to 18 weeks of age but remained unchanged thereafter. The metabolic clearance rate of thyroxine increased significantly through 22 weeks of age 11.83, 2.98, 4.65, 6.32 and 6.90 leter/hr), as did the thyroxine distribution space (2.32, 4.32, 7.10, 90.72 and 10.36 liters. Body size (/.75) was correlated with thyroxine distribution space (r = .49) and thyroxine secretion rate (r = .44).