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Laboratory assessment of thyroid function

Douglas S Ross, MD
Section Editor
David S Cooper, MD
Deputy Editor
Jean E Mulder, MD


Secretion of the thyroid hormones T4 (thyroxine) and T3 (triiodothyronine) is regulated by pituitary thyroid-stimulating hormone (TSH) (figure 1 and figure 2). TSH secretion, in turn, is controlled through negative feedback by thyroid hormones (see "Thyroid hormone synthesis and physiology"). There is a negative log-linear relationship between serum free T4 and TSH concentrations [1]. This means that very small changes in serum free T4 concentrations induce very large reciprocal changes in serum TSH concentrations. As a result, thyroid function is best assessed by measuring serum TSH, assuming steady-state conditions and the absence of pituitary or hypothalamic disease. Nevertheless, direct measurement of serum thyroid hormone levels is still important in many patients since it may be difficult in some patients to be certain about the state of pituitary and hypothalamic function.

This topic review will provide an overview of thyroid function testing. More detailed discussions of serum TSH and free thyroid hormone measurements, abnormalities in thyroid hormone binding proteins, and the effects of drugs and nonthyroidal illness on thyroid function tests are discussed separately. (See "Drug interactions with thyroid hormones" and "Thyroid function in nonthyroidal illness" and "Diagnosis of and screening for hypothyroidism in nonpregnant adults" and "Diagnosis of hyperthyroidism".)


Thyroid function is assessed by one or more of the following tests (table 1):

Serum TSH concentration

Serum total T4 concentration

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Literature review current through: Nov 2017. | This topic last updated: Nov 07, 2017.
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