Hashimoto's thyroiditis and subacute thyroiditis are the most common causes of primary hypothyroidism. Hashimoto's is an autoimmune disorder characterized by elevated TSH and the presence of anti-thyroid antibodies. Subacute thyroiditis causes a transient hypothyroid phase due to thyroid hormone release rather than increased synthesis, with low radioactive iodine uptake and no antibodies present. Graves' disease, the most common cause of hyperthyroidism, results from TSH receptor antibodies that mimic TSH and stimulate increased thyroid hormone production.