Abnormal thyroid function tests are frequently encountered in the evaluation of patients without obvious thyroid disease. Only a fraction of these abnormal tests involve abnormalities in the well understood changes associated with altered levels of thyroid hormone binding globulin (TBG). In the remainder of patients it is necessary to decide whether subtle hypo- or hyperthyroidism exists or whether the abnormal values are somehow spurious. In spite of the ready availability of assays of serum triiodothyronine (T3) and thyrotropin (TSH), and of the ability to test the pituitary-thyroid axis by assessing the response of TSH to thyrotropin releasing hormone (TRH) stimulation, the diagnosis of subtle hypo- or hyperthyroidism is still a problem in clinical endocrinology. This review will consider the new insight into thyroid hormone physiology gained by studies of peripheral metabolism of thyroid hormones during the last ten years and discuss the problems of interpretation of thyroid function tests.
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