Inhibition of hepatic binding of thyroxine by cholecystographic agents

Abstract

Subsequent to studies indicating that cholecystographic agents and sulfobromophthalein (BSP) inhibit uptake of thyroxine (T(4)) by rat liver slices, the effect of such compounds on hepatic storage of T(4) in man has been examined. After intravenous administration of [(125)I]T(4) to five normal subjects, hepatic radioactivity, estimated by external gamma counting, rose to a peak in approximately 4 h and then declined in parallel with serum radioactivity. When a 6-g dose of the cholecystographic agent, tyropanoate (Bilopaque), was administered orally 3 d later, estimated hepatic extravascular radioactivity fell 50-60% within 4 h and then rose toward the pretyropanoate value. Concomitant with the fall in hepatic radioactivity, serum radioactivity rose 57-70%, as did stable T(4) levels in serum, suggesting that hormone discharged from the liver entered the serum. Both uptake of T(4) and discharge by tyropanoate were much less in two patients with liver disease. Ipodate (Oragrafin), 12 g orally in two subjects, caused much smaller changes in hepatic and serum radioactivity. However, ipodate also caused a doubling of the percent free T(4) in the serum as judged by equilibrium dialysis, and the ratio of hepatic radioactivity to free [(125)I]T(4) in serum declined markedly after ipodate, similar to the fall in hepatic:serum (125)I ratios after tyropanoate. BSP, 20 mg/kg i.v. in three subjects, caused a smaller change; the decline in hepatic T4 content averaged 19%. We conclude that these organic anions, tyropanoate, ipodate, and BSP, all can displace T(4) from the liver. The results imply a link between T(4) transport and organic anion transport, and indicate a mechanism for altering hepatic T(4) content in man that could be the site of physiologic regulation or of disease. A method is described whereby analysis of the change in hepatic and plasma radioactivity after tyropanoate could be employed to estimate hepatic T(4) content in diverse clinical circumstances.