Levels of serum thyroid-stimulating hormone in hyperthyroidism.

A modified thyroid-stimulating hormone (TSH) radioimmunoassay with particular sensitivity at the bottom end of the normal range was developed using dog serum in standard tubes and reduced quantities of TSH labeled with radioactive iodine (125I) and antiserum. The standard deviation of B0 points was 1%, implying an assay sensitivity of less than 0.1 microU/mL. This procedure was used to measure serum TSH levels in 32 normal subjects, 18 patients with hyperthyroidism, and seven patients with elevation of serum triiodothyronine (T3) levels. The range of values obtained from normal subjects was less than 0.1 to 4.1 microU/mL, with 97% of the subjects' values greater than 0.2 microU/mL. All of the hyperthyroid patients had values less than or equal to 0.2 microU/mL. Hence, the modified TSH assay was thought to be of use in distinguishing hyperthyroid from normal subjects. The TSH levels in subjects with elevated T3 levels ranged from less than 0.1 to 1.8 microU/mL. These results were thought to indicate heterogeneity within this group and suggest that only certain of these individuals merit the diagnosis of "T3 toxicosis."