Narrow Individual Variations in Serum T 4 and T 3 in Normal Subjects: A Clue to the Understanding of Subclinical Thyroid Disease

High individuality causes laboratory reference ranges to be insensitive to changes in test results that are significant for the individual. We undertook a longitudinal study of variation in thyroid function tests in 16 healthy men with monthly sampling for 12 months using standard procedures. We measured serum T(4), T(3), free T(4) index, and TSH. All individuals had different variations of thyroid function tests (P