Comparison of Bioassay and Immunoassay Measurements of Serum Thyrotropin (TSH) and Study of TSH Levels by Immunoassay of Serum Concentrates*

Immunoassay measurements of serum thyrotropin (TSH) levels in hypothyroid and severely iodine-deficient patients showed good agreement with values obtained by use of the McKenzie mouse bioassay. On direct immunoassay, serum from a small group of euthyroid U. S. or New Zealand subjects contained from <2 to 5.3, mean 3.05, μU/ml TSH. Treatment of the donors with thyroid hormone lowered the TSH level in these subjects only slightly. In contrast, when extracts of serum, concentrated 10–20 times, were assayed, thyroid hormone treatment had a readily demonstrable effect. From assay of extracts the mean serum TSH was 1.01 assay of extracts the mean serum TSH was 1.01 assay of extracts the mean serum TSH was 1.01 μU/ml in the euthyroid subjects and 0.28 μU/ml after thyroid hormone treatment. Similar values were found in pools of serum from normal subjects and thyroxine-treated euthyroid subjects after more extensive concentration and immunoand bioassay. These results indicate that direct immunoassay of serum overestimates serum TSH levels that are less than about 5 μU/ml and that the discrepancy increases as the level of TSH decreases.