Emergency physicians regularly treat patients with thyroid disorders. Until the 1950s, clinical evaluation was the only available diagnostic tool. Since then, increasingly sophisticated laboratory assays have been developed to confirm thyroid pathology. Thyroid physiology, fundamental to interpreting thyroid function tests, is based on a classic negative feedback mechanism involving the hypothalamic-pituitary-thyroid axis. Primary hypothyroidism in developed countries is most commonly caused by Hashimoto's disease. Secondary and tertiary etiologies are uncommon and the result of hypothalamic and pituitary pathology. Clinical presentations range from subclinical disease to myxedema coma. Thyrotoxicosis has many etiologies. A hyperadrenergic state precipitates characteristic signs and symptoms. Thyroid storm and thyrotoxic periodic paralysis are emergent complications. Third generation assays have made thyroid function testing practical for emergency physicians. An ultrasensitive thyroid stimulating hormone level is the most useful. A free thyroxine level is the preferred study for confirming a thyroid disorder. Confounding factors may affect thyroid function interpretation.