Serum Concentrations, Metabolic Clearance Rates, and Production Rates of Reverse Triiodothyronine, Triiodothyronine, and Thyroxine in Starved Rabbits*

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Abstract
In fasted human subjects, serum 3,3'',5''-triiodo-L-thyronine (rT3) concentrations increase and serum 3,5,3''-triiodo-L-thyronine (T3) levels decrease without any significant changes in serum L-thyronine (T4) and TSH [thyrotropin] concentrations. Serum hormone concentrations may be affected by both the production and the degradation of the hormone. Studies were performed to determine serum concentrations, metabolic clearance rates (MCR) and production rates (PR) of rT3, T3 and T4 in starved rabbits. Serum T3 and T4 concentrations were determined by commerical kits, and serum rT3 levels were measured by the specific RIA [radioimmunoassay] developed in the laboratory. The MCR of T4 was determined by single compartmental and those of T3 and rT3 were determined by 3 compartmental analyses. The PR was calculated from MCR multiplied by serum concentration. In starved rabbits, serum T4 concentrations increased from 2.1 to 3.2 .mu.g/100 ml. T4 MCR did not change and T4 PR increased significantly from 3.4 to 4.8 .mu.g/day. Serum T3 levels did not change, but T3 MCR increased from 2.6 to 3.2 l/day and T3 PR increased significantly from 3.5 to 4.3 .mu.g/day. Serum rT3 concentrations increased from 3.3 to 30.8 ng/100 ml but rT3-MCR decreased significantly from 2.3 to 1.4 l/day during starvation. Since the increase in serum rT3 levels was greater than the decrease in rT3 MCR, rT3 PR increased significantly from 78 to 416 ng/day in starved rabbits. In rabbits, the secretion of T4 from the thyroid glands may increase and the production of T3 and rT3 (peripheral conversion of T4 and/or thyroidal secretion) may increase during starvation. The increase of serum rT3 concentrations during starvation is due to both an increase in the production of rT3 and a decrease in the degradation of rT3.