Biochemical Testing of the Thyroid: TSH Really is the Best and, Oftentimes, Only Test Needed — A Review for Primary Care

Abstract

Disorders of thyroid function are common and, often screening, diagnosis, and management is performed by primary care providers. While management of significant biochemical abnormalities is reasonably straight forward, laboratory tests only slightly outside, or even within, the normal range are becoming more difficult to appropriately manage. A large part of this increasing difficulty in appropriate management is caused by patients requesting, and even demanding, certain tests or treatments that may not be indicated. Symptoms of thyroid dysfunction are non-specific and extremely prevalent in the general population. This fact, along with a growing body of information available to patients via the lay press and internet suggesting that traditional testing of thyroid function is not reliable, has fostered some degree of patient mistrust. Increasingly, when a physician informs a patient that their thyroid is not the cause of their symptoms the patient is dissatisfied and even angry. This review aims to clarify the interpretation of normal and mild abnormalities of thyroid function tests by describing pituitary-thyroid physiology and through an in depth review of, arguably, the three most important biochemical tests of thyroid function: TSH, free T4 and anti-TPO antibodies. It is important for primary care providers to have an understanding of the shortcomings and proper interpretation of these tests to be better able to discuss thyroid function with their patients.