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The production and metabolism of 3,5,3'-triiodothyronine and 3,3',5-triiodothyronine in normal and fasting subjects.

The deiodinative pathways of T4 and the production of T3 and rT3 were compared in two groups of normal volunteers on physiological T4 replacement. One group was given a normal diet and the second group was fasted up to 15 days. After 1 week of fasting, the mean serum T3 level fell from 140 to 93 ng/dl, and the mean serum rT3 level rose from 51 to 106 ng/dl. No significant change occurred in the group mean serum concentration of TSH or total or free T4. These changes were most marked during the fifth to seventh day of fasting, but recovered partly toward the baseline values during the second week of fasting. The ratio of mean serum T3 to mean serum rT3 was 2.8 before fasting. It diminished to 0.9 after 1 week of fasting and increased to 1.5 after 2 weeks of fasting. In seven control subjects, the MCRs of T4, T3, and rT3 were 1.27 ± 0.24, 24.0 ± 3.6, and 105 ± 13 liters/day · 70 kg, respectively (mean ± sd). The disposal rates of T4, T3, and rT3 were 140 ± 16, 41 ± 5, and 49 ± 12 μg/day·70 kg, respectively....