Traditionally, it has been thought that the bioavailable fraction of circulating serum hormones, i.e. that which is available for cellular uptake and is physiologically active, is limited to the free (nonprotein bound) hormone. However, recent evidence, based on acute organ uptake of labeled hormone, suggests that the amount of hormone which is bioavailable in vivo may exceed that which is calculated to be free in vitro. To explore the bioavailability of circulating protein-bound thyroid hormones under steady state conditions in vivo, we altered serum thyroid hormone-binding proteins in rats by inducing nephrotic syndrome with puromycin aminonucleoside. Nephrotic rats (serum albumin, 1.1 g/dl) were found to have a marked reduction in serum T4 [2.1 ± 0.2 (SEM) vs. 6.5 ± 0.3 µg/dl; P < 0.01] and an elevation of serum T3 [141 ± 8 vs. 51 ± 2 ng/dl; P < 0.01]. Estimated T4 production rate was normal in nephrotic rats, and the 3- to 4-fold increase in T4 MCR appeared to account for the marked reduction in serum T4. By contrast, increased serum T3 levels in nephrotic rats reflected both a reduction (55%) in T3 MCR and an increased rate of peripheral conversion of T4 to T3. A circulating inhibitor of T4 binding to serum proteins appeared to be present in nephrotic rats. The changes in the various serum components of thyroid hormone [T4-binding prealbumin (TBPA)-bound, albumin-bound, free] produced by nephrotic syndrome were compared with the corresponding changes in indices of thyroid hormone bioavailability (MCR, urinary excretion, hepatic content, TSH suppression, single pass extraction by liver). These comparisons suggested that nephrotic syndrome results in increased bioavailability of circulating T4 and decreased bioavailability of circulating T3. The bioavailable fraction of circulating T3 in vivo seemed to include both free T3 and that which is albumin bound in vitro. The bioavailable fraction of circulating T4 resembled free T4 more than non-TBPA-bound T4 (=albumin bound + free), although a nephrosis-induced increase in bioavailability of TBPA-bound T4 was also possible. We conclude that nephrotic rats have low serum T4, which is related to accelerated T4 clearance, and high serum T3, which is related both to decreased T3 clearance and increased peripheral conversion of T4 to T3. Under steady state conditions in vivo, bioavailable circulating T3 appears to include both free T3 and the T3 that is bound to albumin in vitro.
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