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Test Update

An Algorithmic Approach to Thyroid Function Testing

March 2009

Blood tests to measure thyroid function (TSH, T4, and T3) are readily available and widely used. This diagnostic testing algorithm outlines an efficient evaluation of thyroid status through a series of tests. By using an algorithmic approach to thyroid testing, physicians can make timely diagnoses, which, in turn, can significantly reduce the cost of care by eliminating downstream expensive redraws, repeat orders, return office visits, extended hospital stays, and unnecessary physician time. In Mayo Medical Laboratories’ test #83633 Thyroid Function Cascade, Serum all appropriate tests are performed automatically (individual thyroid function assays may also be ordered separately). See Mayo’s recommended approach to thyroid function testing in the algorithm in this issue.

Initial evaluation of thyroid function begins with sensitive thyroid-stimulating hormone (s-TSH; #8939 Thyroid-Stimulating Hormone - Sensitive [s-TSH], Serum), a highly effective screening assay. Increased s-TSH indicates that the thyroid gland is failing because of a problem that is directly affecting the thyroid (primary hypothyroidism) and, thus, producing inadequate thyroid hormone; suppressed s-TSH indicates excess thyroid hormone is being produced by an overactive thyroid (hyperthyroidism). In our experience, the majority of people tested will have a normal s-TSH level, no further testing is necessary, and no further costs are incurred. However, when the s-TSH result is abnormal, appropriate follow-up tests are automatically performed.

If s-TSH is <0.3 mIU/L or >5.0 mIU/L, free thyroxine (FT4) is performed (#8725 T4 [Thyroxine], Free, Serum). The supplemental measurement of FT4 in patients with abnormal s-TSH measurements allows better assessment of the how the thyroid gland is functioning. A low TSH with an elevated FT4 is found in individuals who have hyperthyroidism. Conversely, an elevated s-TSH and low FT4 indicates primary hypothyroidism due to disease in the thyroid gland. A low s-TSH and low FT4 indicates hypothyroidism due to a problem involving the pituitary gland.

Serum triiodothyronine (T3) levels often are depressed in sick and hospitalized patients, caused in part by the biochemical shift to the production of reverse T3 (euthyroid sick syndrome). Therefore, T3 generally is not a reliable predictor of hypothyroidism. However, in a small subset of hyperthyroid patients, hyperthyroidism may be caused by overproduction of T3 (T3 toxicosis). To help diagnose and monitor this subgroup, T3 (#8613 T3 [Triiodothyronine], Total, Serum) is measured on all specimens with suppressed s-TSH and normal FT4 concentrations.

In some patients with hypothyroidism or hyperthyroidism, lymphocytes make antibodies against the thyroid that either stimulate or damage the gland (autoimmune thyroid disease). Measuring levels of thyroid antibodies may help diagnose the cause of the thyroid problems. Detectable concentrations of antithyroperoxidase antibodies (anti-TPO) are observed in patients with Hashimoto thyroiditis, idiopathic myxedema, and Graves disease. Anti-TPO antibodies (#81765 Thyroperoxidase [TPO] Antibodies, Serum) are measured in all specimens with elevated s-TSH concentrations.

Mayo Clinic is internationally renowned for the treatment of thyroid problems. Clinical consultation with Mayo endocrinologists and pathologists is available through Mayo Medical Laboratories to work together to develop the best treatment plan for each patient.

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