Comparison of serum thyrotrophin concentrations determined by a third generation assay in patients with various types of overt and subclinical thyrotoxicosis

OBJECTIVE Recent increases in the sensitivity of methods used to measure TS H, especially third generation assays, have enabled separation of partial f rom complete pituitary suppression in patients with thyrotoxicosis, We have investigated the use of a sensitive chemiluminescent enzymeimmunoassay in the differential diagnosis of thyrotoxicosis, DESIGN AND PATIENTS Serum TSH concentrations were determined by chemilumine scent enzymeimmunoassay in patients with various types of overt and subclin ical thyrotoxicosis, RESULTS The assay was highly sensitive with an analytical sensitivity of 0. 0016 mU/l. Among 45 hyperthyroid patients with untreated Graves' disease, 3 7 (82.2%) showed undetectably low levels (<0.002 mU/l), Serum TSH in the re maining 8 patients (17.8%) was 0.003-0.005 mU/l. In contrast, TSH was undet ectably low in only 5 (20.0%) of 20 patients with painless thyroiditis and in 2 (12.5%) of 16 patients with subacute thyroiditis. Eleven (55.0%) with painless thyroiditis and 12 (75.0%) with subacute thyroiditis had TSH value s greater than 0.005 mU/l (0.006-0.032 and 0.006-0.228 mU/l, respectively; normal range 0.5-5.0 mU/l), Serum TSH levels were subnormal in 10 of 12 pat ients with euthyroid ophthalmic Graves' disease, including 4 with undetecta bly low levels. Among 11 patients with an autonomously functioning thyroid nodule 6, including 1 with normal free T4 and free T3 and 2 with normal fre e T3, showed TSH values less than 0.002 mU/l. No significant correlation wa s observed between serum free T4 or free T3 and TSH concentrations in thyro toxic patients. Together with the incomplete suppression of TSH observed in those with destructive thyroiditis, this suggests that the grade of TSH su ppression was influenced by the duration of illness at the time of blood sa mpling. CONCLUSION The third-generation TSH assay is useful for the differential di agnosis of various types of thyrotoxicosis, especially between Graves' dise ase and destructive thyroiditis.