Fecal and urinary excretion of six iodothyronines in the rat

Fecal and urinary excretion rates of six iodothyronines were assessed in the rat maintained under normal steady state physiological conditions, to gain a more comprehensive understanding of the mechanisms of control of normal thyroid hormone economy and metabolism. Groups of young adult male rats were injected with trace doses of T4, T3, rT3, 3,3'-diiodothyronine (T2), 3',5'-T2, or 3'-monoiodothyronine, each labeled with 125I, and feces and urine were collected separately for up to 10 days. Pooled fecal pellets were homogenized in saline, extracted in ethanol, evaporated under vacuum, and reconstituted in NaOH. Fecal extracts and urine were chromatographed on Sephadex G25 columns under conditions providing quantitative separations of components of interest. A new technique was also developed, based on a model of the in vitro extraction and measurement process, to correct chromatographic results for possible variable recoveries and possible artifactious degradation of radioactively labeled components. No iodothyronines or their conjugates were excreted in urine; all radioactivity was in the form of iodide. In feces, about 30% of the [125I]T3 injected was excreted as T3; and 24% of the [125I]T4 injected was excreted as T4, plus 4% as T3. Together, these results imply that about 24% of endogenous T4 production is excreted as T4 and 76% is irreversibly metabolized; and for T3, about 30% of endogenous T3 production is excreted as T3 and 70% is degraded. For the nonhormonal iodothyronines, about 6% of injected monoiodothyronine, 3% of injected 3',5'-T2, 2% of injected 3,3'-T2, and less than 1% of injected rT3 were excreted in feces as such, indicating that these substances are nearly completely deiodinated in vivo. Very little (1-7%) iodide was excreted as such in feces, which also were devoid of measurable conjugates. An open question is whether the substantial wastage of thyroid hormones in feces represents poor hormone economy in the usually accepted sense or a functional property of overall thyroid hormone regulation.