The distribution of241 Am in the thyroid glands of three dogs after intravenous injection was studied. The glands were removed, frozen immediately in liquid N2, and counted. Later the thyroid tissue was homogenized and separated into fractions by differential centrifugation. Eighty-five to ninety per cent of the241 Am was found with the crude fibrous residue. Combining this fibrous material with cell nuclei and membranes accounted for more than 90% of the total activity found in the thyroid. Only very small quantities were found with microsomes, mitochondria, and in the 105,000 × g supernatant. It is concluded that241 Am is selectively bound by the connective tissue elements of the thyroid. The concentration of241 Am in thyroid connective tissue was found to be 337 times that of a sample of subcutaneous loose connective tissue and 4 times that of connective tissue derived from the liver.

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