The effect of thyroxine administration on intestinal calcium absorption and calcium binding protein activity in the chick.

Thyroxine (T4) injected subcutaneously into 3-week-old chicks (30 mug/kg body weight daily for 8 days) reduced the absorption of calcium from in vivo ligated loops of duodenum and the duodenal calcium binding protein (CaBP) activity measured by the Chelex ion exchange method. Chicks fed either a commercial diet or an artificial diet containing 0.5% or 1.0% calcium showed similar response to treatment with T4. A significant effect of T4 on either calcium absorption of CaBP activity could not be demonstrated in chicks fed a high calcium diet (2.0%). The possible mechanisms of action of T4 on calcium absorption are discussed.