Absorption of thyroxine from the intestine of rats

Abstract

Within 1 hr. less than 40% of a L-thyroxine dose was absorbed from washed loops of rat intestine and this was independent of the administered doses from 0.3 to 30 [mu]g thyroxine. A greater absorption of thyroxlne occurred from a washed loop of large intestine than from a washed loop of small intestine. Normal intestinal contents markedly reduced the absorption of thyroxine from intestinal loops; the absorption was greater in fasting rats than in rats fed a high-residue diet. Absorption of thyroxlne from germfree rat colon was significantly greater than that from conventional rat colon. It is concluded that endogenous plasma proteins are continually transported into the lumen of the gastrointestinal tract and these proteins, together with insoluble dietary factors and intestinal flora, limit thyroxine absorption. It is suggested that free thyroxine in the gut lumen is the primary fraction of thyroxlne available for the absorption from the intestine.