The effects of nonthyroid disease and drugs on thyroid function tests

Abstract

Systemic nonthyroid illness has effects on thyroid hormone secretion, transport, and metabolism. The magnitude of the abnormality in thyroid function tests depends more on the severity than on the nature of the illness. The most common pattern in this 'euthyroid-sick syndrome' is a low serum T3, normal or low total T4, and normal free T4 concentration. The serum thyroid-stimulating hormone concentration is usually normal but may be slightly elevated or even suppressed. Certain commonly used drugs also influence thyroid function indices. Knowledge of these effects is important for correct assessment of thyroid status in the patient with nonthyroid illness.