Enhanced thyroxine metabolism following cutaneous application of microscope immersion oil

Microscope immersion oil, applied to the skin of rats or injected intraperitoneally, led to a substantial increase in the biliary excretion of intravenously injected 125-I-thyroxine (T4), elevated the bile: plasma 125-I ratio, enhanced the biliary clearance rate of plasma 125-I-T-4 and increased the proportion of biliary 125-I present as T-4-glucuronide. Immersion oil applied to the skin was at least as effective as injected intraperitoneally. Skin application also elevated bile flow, liver weight, thyroid 131-I uptake and Sephadex uptake of serum 125-I-triiodothyronine (indicating reduced binding of thyroid hormone to serum proteins) and depressed serum T-4 concentrations. These effects were similar to those previously observed after intraperitoneal injection of a polychlorinated biphenyl mixture.