Thyroid dysfunction meets many criteria for a condition justifying population screening:

1. The prevalences of various forms of thyroid dysfunction are substantial.
2. Overt hypothyroidism and hyperthyroidism have well-established clinical consequences. Even mild hypothyroidism can progress to overt hypothyroidism,¹, ⁸ particularly in patients with antithyroid antibodies or previous thyroid irradiation. (Mild hypothyroidism refers to patients in whom there is elevation of the serum thyrotropin [thyroid-stimulating hormone (TSH)] concentration in association with a normal serum free thyroxine [(FT₄]) concentration. This state is also termed subclinical hypothyroidism, compensated hypothyroidism, decreased thyroid reserve, and prehypothyroidism.) Mild hypothyroidism can also be associated with reversible hypercholesterolemia,^9-12 particularly when the serum TSH concentration is greater than 10 mIU/L, and, in some patients, with reversible symptoms¹³, ¹⁴ and cognitive dysfunction.¹⁵, ¹⁶ Mild (subclinical) hyperthyroidism has been associated with a higher incidence of atrial fibrillation in older persons¹⁷; reduced bone mineral density,¹⁸, ¹⁹ particularly in postmenopausal women; and symptoms (eg, palpitations) in some patients.²⁰
3. The serum TSH assay is an accurate, widely available, safe, and relatively inexpensive diagnostic test for all common forms of hypothyroidism and hyperthyroidism.²¹
4. There are effective therapies for both hypothyroidism and hyperthyroidism for patients in whom treatment is indicated.

Screening of all newborn children for hypothyroidism is already a widely accepted and legislatively mandated practice. In addition, serum TSH measurement in adults every 5 years has been shown by decision analysis to have equivalent or more favorable cost-effectiveness in comparison with other widely accepted disease detection strategies,²² for example, for hypertension, breast cancer, and hypercholesterolemia. The cost-effectiveness of screening is more favorable in women and older persons and is strongly influenced by the cost of TSH measurement. Consequently, it is recommended that all adults have their serum TSH concentration measured beginning at age 35 years and every 5 years thereafter, the interval at which a periodic health examination has been advocated by the US Preventive Services Task Force.²³ More frequent screening may be appropriate in individuals at higher risk of developing thyroid dysfunction.