Relative Rates of Biological Decay of Thyroidal I 131 in Rat and Guinea Pig. ∗

Abstract

Summary and Conclusions

1. The rate of biological decay of thyroidal I₁₃₁ was compared in rat and guinea pig by determining the reduction in radioactivity over the gland utilizing the in vivo detection technic. The biological half-life of radioiodine was estimated to be 3.5 and 26 days for the rat and guinea pig respectively. 2. The results are considered to suggest that thyroid hormone secretion in the guinea pig normally proceeds at a much slower rate than in the rat. Consideration of other parameters of pituitary-thyroid function for both species supports this view.