Thyroid diseases are common, and most can be safely and effectively managed in primary care. Two of the most common reasons for thyroid function testing are fatigue and obesity, but the vast majority of affected patients do not have hypothyroidism. There is no plausible basis for the assertion that hypothyroidism commonly occurs despite normal thyroid function tests. In primary hypothyroidism all patients, except the elderly and those with ischaemic heart disease, can safely be started on a full replacement dose of thyroxine; the aim is to restore thyroid stimulating hormone (TSH) to normal. Triiodothyronine (T3) has no role in the treatment of primary hypothyroidism. Subclinical thyroid disease should not be treated except in certain well defined situations. Its main importance lies in the increased risk of progression to overt thyroid disease. The development of hyperthyroidism is easily overlooked, and it is important to maintain a high index of suspicion, especially in the elderly. The most common causes are Graves' disease and thyroiditis (especially postpartum), and in the elderly toxic nodular goitre and amiodarone. Patients taking amiodarone should have their thyroid function checked every 6 months. Patients with overt hyperthyroidism should be referred for specialist management; beta-blockers and sometimes anti-thyroid drugs may be initiated in primary care. Most thyroid nodules, especially those detected incidentally on ultrasound scanning, are benign. Indications for referral include newly occurring nodules >1cm in diameter, painful nodules, and nodules that are increasing in size.