• Use Thyroid function test. Investigation of low T4 (RIA) 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.

• The highly sensitive TSH assays can be considered as a test for thyroid disease. A result within the accepted reference interval provides strong evidence for euthyroidism.

• Unsuspected increase in the level of serum TSH is not uncommon in elderly subjects. A study by Sawin et al found that 22 of 344 (5.9%) healthy persons older than age 60 had TSH level >10 mIU/L; 10 of the 22 had low T4 and FT4 index. Elderly hypothyroid individuals may have minimal recognizable clinical symptoms of thyroid deficiency.10 TSH is the single most sensitive test for primary hypothyroidism. If there is clear evidence for hypothyroidism and the TSH is not elevated, hypopituitarism should be considered (secondary hypothyroidism).

• TSH levels have been elevated or inappropriately detectable for high thyroid hormone levels in some patients with thyrotropin-secreting pituitary adenomas. Delay in diagnosis of these tumors may lead to visual compromise. The effects of such neoplasms can be misdiagnosed as those of primary hyperthyroidism.

• Until the late 1980s, TSH assays were not sufficiently sensitive to distinguish hyperthyroidism from euthyroid (normal) subjects. The new generation of ultrasensitive TSH immunoassays have provided a far more effective diagnostic separation of thyrotoxicosis from euthyroidism.

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