Thyroid-stimulating hormone (TSH), serum thyroxine (T4) and triiodothyronine (T3) in sera of 100 patients with primary thyroid failure were measured by radioimmunoassay. Effective thyroxine ratio (ETR) was also measured in 69 of these sera. TSH was elevated in all, with a mean of 76.7 +/- 55 micromicron/ml (range 11 to 140 micromicron/ml, and was used to confirm the diagnosis. Mean T4 and T3 levels in the group were 1.8 +/- 1.5 microgram/100 ml (range less than 0.2 to 7.0 microgram/100 u.) and 76.7 +/- 76 ng/100 ml (range less than 10 to 600 ng/100 ml), respectively. Mean ETR was 0.81 +/- 0.05. A moderate inverse relationship was observed between TSH and T4 (r = 0.73), in contrast to TSH and T3 and TSH and ETR, which showed comparatively poor relationships (r = 0.41 and 0.43). This observation, in addition to the finding that 17 patients had subnormal T4 but normal or elevated T3, suggests that normal T3 levels alone may not be sufficient to maintain euthyroidism. In contrast, there were only three clinically hypothyroid patients who had elevated TSH, normal T4, but subnormal T3 levels. This study indicates that T4 and T3 may function together to maintain euthyroidism, and that in addition to serum TSH, T4 determination has more diagnostic value than serum T3 or ETR in these patients.