In vitro thyroid function tests were studied in 30 patients on regular hemodialysis. In addition, 8 patients (one not yet dialyzed) underwent perchlorate discharge testing and the salivary to plasma ratio of 131I was measured to identify defects in iodine uptake or organification. When compared with 35 healthy controls, uremic patients had lower total T4 (5.8 +/- 0.4 vs 8.6 +/- 0.3 micrograms/100 ml) and total T3 (81 +/- 6 vs 124 +/- 5 ng/100 ml) but T3RU was higher in dialysis patients (35.5 +/- 0.9 vs 28 +/- 0.5%, P less than 0.01). The TSH levels were not different from those of controls. The RAI uptake 1 h after perchlorate was significantly higher than the control valve (0.6 +/- 0.14 vs 0.06 +/- 0.04, P less than 0.05). Similarly, the salivary to plasma ratio of 131I was higher in patients with chronic renal failure (70 +/- 10:1 vs 40 +/- 4:1). It is concluded that there is altered iodine trapping in CRF patients but iodine organification appears to be normal.