[Peripheral conversion of T4 and T3 in man].

Our investigations of the peripheral metabolism of T4 and T3 in man demonstrate clearly that T3 is secreted by the thyroid gland and is peripherally formed by conversion from T4. The calculation of the disposal rates indicate furthermore that 40% of the daily metabolised T4 is converted to T3. In this way more than 60% of the daily production of T3 results from peripheral conversion of T4. Our results support the hypothesis that the physiological action of the thyroid hormones is effected mainly by T3.