Technology advances have given the clinician and the laboratory a large armamentarium of tests permitting measurement of virtually every aspect of thyroid function. Ready availability at relatively low cost from clinical laboratories allowed these tests to be applied widely to patients. As this occurred, it became evident that abnormal test results were due to nonthyroidal illness in euthyroid sick patients far more often than true indicators of a thyroid disorder. Thus, despite the elegance of today's thyroid function tests, their proper use and interpretation still require careful observation of the patient's symptoms and signs and assessment of the overall clinical situation of the patient. Despite the proliferation of thyroid function tests, just three tests, free-T4 index, free-T3 index, and TSH, suffice for the evaluation of the thyroid status of all but a very few patients. In the form of an algorithm we have outlined a logical sequence for the development and interpretation of thyroid function tests that prompts the clinician to integrate the results with clinical assessment.