Relative Rates of Biological Decay of Thyroidal I131 in Rat and Guinea Pig.

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Abstract
The amt. of I131 contained in the thyroid gland of guinea pigs and rats receiving a single intraperit. dose of I131 (25-40 and 25 [mu]c., respectively) was measured by means of an external gamma radiation-detection technique. Counts were made in ether anaesthetized animals (daily for 8 days in rats and every 3-4 days for 3 weeks in guinea pigs) under conditions of constant geometry and with corrections made for extra-thyroidal activity (calculated from values obtained in thyroidectomized animals) and for isotopic decay. Maximal uptake of radioiodine by the thyroid (48-72 hrs. for guinea pigs, 24 hrs. for rats) was followed by a slow progressive decline in gland radioactivity. The graphed relationship of the counts as percentage of the maximum plotted on semilogarithmic paper against time was approx. linear. The biological half life of I131 in the guinea pig thyroid calculated from this curve was found to be about 26 days whereas that of the rat was approx. 3.5 days. The findings suggest that the rate of thyroid hormone secretion in the normal guinea pig is much slower than in the rat, which, together with consideration of other parameters supports the view that in the guinea pig the pituitary-thyroid system functions at a relatively low level.