The presence of a high concentration of thyroxine in thyroid epithelial cells.

Swine thyroid epithelial cells were isolated and the intracellular concentration of thyroxine was measured by radioimmunoassay and high pressure liquid chromatography. The estimated concentrations of intracellular thyroxine were 1.89 X 10(-8) micrograms/cell by radioimmunoassay, and 0.708 X 10(-8) micrograms/cell by high pressure liquid chromatography, which were equivalent to 4.39 X 10(-5) M and 1.78 X 10(-5) M, respectively. These levels were about a thousand-fold higher than that in the circulation (3.18 +/- 0.59 microgram/dl, 3.98 X 10(-8) M).