Hypothyroidism

Hashimoto’s thyroiditis
Iodine deficiency (rare in developed world)
Thyroid surgery, irradiation, or radioiodine
Subacute (de Quervain’s) thyroiditis (after initial hyperthyroidism)
Severe illness
Drugs causing hypothyroidism: lithium, amiodarone, propylthiouracil, methimazole, phenylbutazone, sulfonamides, interferon ?
Riedel’s thyroiditis
Deficient pituitary thyroid-stimulating hormone (TSH)
"Subclinical" hypothyroidism: high TSH, clinically euthyroid, normal T4