Primary hypothyroidism is usually the result of Hashimoto's thyroiditis, thyroidectomy for hyperthyroidism, goiter or cancer, or radioactive iodine therapy for hyperthyroidism.

Hypothyroidism is usually the result of Hashimoto's thyroiditis, thyroidectomy for hyperthyroidism or cancer, or radioactive iodine therapy. Treatment of hypothyroidism with replacement doses of thyroid hormone is usually lifelong. The drug of choice for thyroid replacement therapy is levothyroxine. Liothyronine and dessicated porcine thyroid are also marketed for this indication.