Use: Thyroid function test. Investigation of low thyroxine (T4) result; the differential diagnosis of primary hypothyroidism from normal, and the differential diagnosis of primary hypothyroidism from pituitary/hypothalamic hypothyroidism. TSH is high in primary hypothyroidism. Low TSH occurs in hyperthyroidism. Evaluation of therapy in hypothyroid patients receiving various thyroid hormone preparations: Low values are found in states of excessive thyroid replacement. Normal result on a sensitive TSH assay is acceptable evidence of adequate thyroid replacement.Follow-up of patients who have had hyperthyroidism treated with radioiodine or surgery. Follow-up low T4 newborn results.This third-generation TSH assay can be considered a test for thyroid disease. A result within the accepted reference interval provides strong evidence for euthyroidism.