Externally Peer Reviewed by Mosby Overview Definition.
Primary hypothyroidism refers to a thyroid hormone deficiency as a result of thyroid gland disease.
Secondary hypothyroidism results from TSH deficiency. Etiology Without thyroid enlargement. Commonly caused by 131 I therapy, thyroidectomy for hyperthyroidism. The second most common cause is idiopathic hypothyroidism. Developmental defects and TSH or TRH deficiency are less common causes.