World-wide, iodine deficiency remains the most significant cause of hypothyroidism. Thyroid autoimmune disease (Hashimoto’s thyroiditis) [2] is the most common cause of spontaneous primary hypothyroidism in regions with normal or even moderately low iodine intake. The degree of hypothyroidism is most often mild (subclinical) and slowly progressive in Hashimoto’s thyroiditis, but all degrees of hypothyroidism can be seen and sudden onset is sometimes seen. Many patients with Hashimoto’s thyroiditis have a rubbery goiter, diffusely hypoechoic and vascular on neck ultrasound. In some cases, the thyroid becomes completely atrophic and cannot be palpated. Thyroid peroxidase antibody tests are almost universally positive in Hashimoto’s thyroiditis. Almost all other forms of primary hypothyroidism are iatrogenic, caused by either thyroid surgery, or radioactive iodine treatment of hyperthyroidism. Inflammatory or toxic forms of hypothyroidism can occur as a consequence of medications (amiodarone) or viral insults (subacute thyroiditis) and may result in transient forms of primary hypothyroidism. Central hypothyroidism is vastly less common than primary and usually occurs in the setting of multiple pituitary hormone insufficiencies.