Primary hypothyroidism.

Volume 91, Number 6 www.aafp.org/afp American Family Physician 359 Primary hypothyroidism is defined as low levels of blood thyroid hormone due to destruction of the thyroid gland. This destruction is usually caused by autoimmunity or an intervention such as surgery, radioiodine, or radiation. • It can be classified as clinical (overt) when diagnosed by characteristic clinical features, raised levels of thyroid-stimulating hormone (TSH), and reduced levels of thyroxine (T4); or subclinical when serum TSH is raised, but serum T4 is normal, and there are no symptoms of thyroid dysfunction. • Hypothyroidism is six times more common in women, affecting up to 40 in 10,000 each year (compared with six in 10,000 men). There is consensus that levothyroxine is effective in treating clinical (overt) hypothyroidism, but evidence is sparse. • Treatment can lead to hyperthyroidism, reduction of bone mass in postmenopausal women, and increased risk of atrial fibrillation if the person is overtreated. • We found no evidence from randomized controlled trials that levothyroxine plus liothyronine improves symptoms more than levothyroxine alone. We do not know how effective levothyroxine is in treating persons with subclinical hypothyroidism, as trials have been too small to detect any clinically relevant improvements in outcomes.