What is the most appropriate single screening test for thyroid disease?

The TSH assay is the most appropriate single screening test for thyroid disease because it effectively reflects the functioning of the thyroid gland. Thyroid-stimulating hormone (TSH) is produced by the pituitary gland and regulates the production of thyroid hormones, T4 and T3, from the thyroid gland itself.

In individuals with thyroid dysfunction, changes in TSH levels can indicate whether the thyroid is underactive (hypothyroidism) or overactive (hyperthyroidism). Typically, a high TSH level suggests that the thyroid is not producing enough hormones, while a low TSH level indicates excess thyroid hormone production. Because of its sensitivity and specificity in detecting thyroid dysfunction, TSH is often the first test ordered and is considered the gold standard for screening.

While other tests such as total T4 or free thyroxine index measure thyroid hormone levels, they cannot be relied upon as standalone screening tests because their results can be influenced by various factors, including changes in protein binding and acute illness. Serum T3 assays can sometimes be misleading, particularly in subclinical conditions, because T3 levels may remain normal even when T4 levels and symptoms suggest dysfunction. Thus, the TSH assay remains the most appropriate choice for initial evaluation of thyroid health.