Determination of Iodothyronine Absorption and Conversion of L-Thyroxine (T₄) to L-Triiodothyronine (T₃) using Turnover Rate Techniques

The absorption of L-thyroxine (T₄) and L-triiodothyronine (T₃) and the fractional rate of conversion of T₄ to T₃ were determined from the turnover rates of T₄ and T₃ in seven patients without endogenous thyroid function during separate treatment periods with these iodothyronines. Serum T₃ concentration was measured by a radioimmunoassay procedure in which the iodothyronines are separated from the plasma proteins before incubation with anti-T₃ antibody. Metabolic clearance rates were calculated by an integral (noncompartmental) approach since the use of single compartment kinetics led to a 40% overestimation of the metabolic clearance rate of T₃. Based on the amount of hormone ingested and the observed hormonal turnover rates, the absorption of T₄ and T₃ (iodothyronine turnover/iodothyronine ingested) in man could be estimated. Absorption of T₃ was complete in three subjects but decreased to 43% in a fourth who was suffering from mild congestive heart failure. Mean T₄ absorption was 48.0±2.6% (SEM) for seven subjects. The mean fractional rate of T₄ to T₃ conversion determined during T₄ replacement therapy (T₃ turnover/T₄ turnover) was 42.6% (range 30.7-50.8%). Thus, approximately one-half of the T₄ which was deiodinated was converted to T₃ suggesting that monodeiodination is an obligatory step in the peripheral metabolism of T₄. Calculations based on these results together with other available data suggest that under normal physiologic circumstances the major portion of the T₃ pool is derived from monodeiodination of T₄.

Click on an image below to see the page. View PDF of the complete article

page 805

page 806

page 807

page 808

page 809

page 810

page 811