Measurement of serum thyroid hormone and TSH levels provide diagnostic information in the majority of patients with thyroid dysfunction. The test strategy in hyperthyroidism differs from that in hypothyroidism. Serum T4 is a good test for hyperthyroidism in patients with normal thyroid hormone-binding protein levels. When binding proteins are abnormal serum free T4 is a much more accurate test for hyperthyroidism than serum T4. Serum T3 and the TSH response to TRH are useful tests for the early diagnosis of hyperthyroidism. Serum TSH is a very sensitive indicator of primary hypothyroidism rising already at the subclinical stage of the disease. Serum T4 and free T4, but not serum T3, are useful for the verification of clinical hypothyroidism. Determination of the TRH-stimulated TSH level is important for the differential diagnosis of pituitary and hypothalamic hypothyroidism. It is imperative to recognize that thyroid tests are often abnormal in various non-thyroidal diseases and that administration of drugs can affect these tests. Serum rT3 is of some value for the assessment of thyroid function in patients with non-thyroidal disease.