A radioimmunoassay procedure to measure triiodothyronine (T
3) in unextracted urine is described. One hundred, μl of T
3 standards or samples were incubated with 100, μl of T
3 antiserum (1:40, 000), 100μl of tracer
125I-T
3 and 700μl of 0.05M borate buffer, pH 8.6. Separation of free from bound antigen was achieved by dextran coated charcoal. The cross-reaction of L-T
3 with L-T
4 in this assay system was less than 0.2%. Dilution of high T
3 urine was parallel to the standard curve. Recovery was 101±8%. Coefficients of variation were 3-8% within each assay and 13% between assays. Euthyroid subjects excreted 0.81±0.39μg (mean±S. D. ) in 24hr. Similar values were obtained in pregnant women, euthyroid patients with chronic thyroiditis, and a patient with thyroxine-binding globulin deficiency. Excretion of urinary T
3 was high (7.48±3.32μg/24h) in patients with hyperthyroidism, and low (0.14±0.15μg/24hr) in patients with hypothyroidism. Urinary T
3 excretion was almost undetectable in terminal renal failure. Positive correlations were found between urinary T
3 and serum total T
3 (
r=0.89) and especially serum free T
3 (
r=0.97). A positive correlation was also found between urinary T
3 and creatinine clearance (
r=0.63). Mean urinary clearance of
T3(
CT3=Urinary
T3 excretion/serum free
T3 was 166ml/min and was significantly higher than their mean creatinine clearance (110ml/min). The measurement of T
3 in urine is reliable and easy to perform, and may allow a new approach to the understanding of thyroid hormone metabolism.
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