Metabolic Clearance and Production Rates of 3′,5′-Diiodothyronine and 3,3′-Diiodothyronine in Man*

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Abstract
MCRs and production rates (PRs) of 3′,5′-diiodothyronine (3′,5′-T2), 3,3′-diiodothyronine (3,3′-T2), and rT3 were determined in seven, five, and four normal subjects, respectively. The constant infusion technique was used to measure the MCR; the PR was calculated as the product of MCR and the concentration of iodothyronine as measured by RIA. The results of measurements of MCR and PR were adjusted to 70 kg BW. In the case of 3′,5′-T2, the serum concentration (mean ± SE) was 6.5 ± 0.4 ng/100 ml, the MCR was 159 ± 21 liters/day, and the PR was 10.9 ± 1.5 μg/day. The serum concentration of 3,3′-T2 was 4.0 ± 0.6 ng/100 ml, while its MCR was 560 ± 59 liters/day and the PR was 23 ± 4.5 μg/day. The serum concentration of rT3 was 39 ± 8.7 ng/100 ml, MCR was 131 ± 27 liters/day, and the PR was 44.8 ± 6.5 μg/day. Additionally, serum concentrations of 3′,5′-T2 and 3,3′-T2 were measured before and after oral administration of 300 μg L-rT3 to normal subjects. Serum 3′,5′-T2 levels increased maximally at 1 h to 45 ± 12% above baseline, whereas serum 3,3′-T2 levels increased maximally at 2 h to 185 ± 71% above baseline. We conclude that 3′,5′-T2 and 3,3′-T2 are produced in substantial quantities in normal man and that peripheral metabolism of rT3 is an important source of both T2s. In addition, the sum of the PRs of the T2s measured is substantially less than what would be expected if all of the rT3 and T3 produced daily were metabolized only to 3′,5′-T2 and 3,3′-T2. Therefore, it seems predictable that T3 is metabolized to 3,5-T2 as well as to 3,3′-T2. (J Clin Endocrinol Metab 48: 297, 1979)