Enhanced thyroxine metabolism and high uptake goiters in rats after a single dose of 2,3,7,8-tetrachlorodibenzo-p-dioxin.

Nine days after treatment of rats with a single dose (25 microgram/kg) of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), the 1 h biliary excretion of [125I]-thyroxine (T4) was increased 4-fold and the [125I]T4 bile:plasma ratio and the biliary clearance rate of plasma [125I]T4 were increased 10-fold. The proportion of biliary 125I present as T4-glucuronide was larger in TCDD-treated rats. Unexpectedly, TCDD did not influence the 30 min biliary excretion of [125I]triiodothyronine (T3), the [125I]T3 bile:plasma ratio, and the biliary clearance rate of plasma [125I]T3. TCDD also elevated serum thyrotropin concentrations, produced goiters and increased thyroid 131I uptake. Serum T4 concentrations were reduced to half of normal in TCDD-treated animals, but their serum T3 levels were elevated. Sephadex uptake of serum [125I]T3 was reduced in the TCDD group.