The renal clearance of thyroid hormones in the isolated perfused rat kidney

The isolated perfused rat kidney was used to study renal handling of thyroid hormones. Substrate enriched protein free perfusates were chosen which allowed the kidneys to perform a normal glomerular filtration rate of 1.1 ml/g kidney x min and enabled accurate estimation of filtered load and tubular handling of free thyroid hormones without interference from the presence of hormone binding substances. Under these conditions we found a high tubular reabsorption capacity for both T3 and T4. This reabsorption capacity was unsaturable, even when the filtered hormone load was far above (x 100) the physiological range.