The determination of thyroxine in the thyroid gland of the rat.

Most procedures used for the separation of thyroxine from other iodinecontaining compounds of the thyroid gland are based on either the method of Harington and Randall (I) or that of Leland and Foster (2). In the former the thyroxine is separated from diiodotyrosine by precipitation in a slightly acid medium, whereas in Leland and Foster’s method thyroxine is extracted with butyl alcohol from an alkaline hydrolysate of thyroid tissue. Leland and Foster observed that their method yielded much lower values for the thyroxine content of thyroid material than did the method of Harington and Randall; they attributed this to the presence of diiodotyrosine in the acid-insoluble fraction. Blau (3) modified the method of Leland and Foster by adjusting the alkaline hydrolysate to pH 3.5 to 4.0 before its extraction with butyl alcohol. The thyroxine values obtained by this modification are 10 to 20 per cent higher than those obtained by the original Leland and Foster method, but still much lower than those obtained by the acid precipitation procedure of Harington and Randall (4). The methods referred to above were designed for the analysis of relatively large amounts of thyroid tissue. Foster (5) subsequently adapted the butyl alcohol extraction procedure for the determination of the thyroxine content of the guinea pig thyroid. In the present communication a modification of Foster’s method is described. Data on the thyroxine content of the rat thyroid are also presented.