Hypothyroidism may be divided into two categories:

Primary Hypothyroidism: Generally hypothyroidism occurs due to a problem (pathology) in the thyroid gland itself, in which the gland produces much less thyroid hormones than normal. Causes may include:

  • Hashimoto’s thyroiditis (an autoimmune disorder where the body’s own immune system starts attacking the thyroid tissue)
  • Thyroid failure following iodine therapy or surgical treatment of hyperthyroidism
  • Goitrous hypothyroidism: caused due to prolonged intake of iodine deficient diet.
  • Congenital hypothyroidism: Inborn (from birth) thyroid defect

Secondary Hypothyroidism: In a few cases, hypothyroidism may also result due to a problem in the hypothalamus or the pituitary rather than the thyroid itself. Normally the hypothalamus sends signals to the pituitary (known as the ‘master gland’) to secrete TSH (Thyroid Stimulating Hormone), which in turn stimulates the thyroid gland to secrete thyroid hormones (T3 and T4). In diseases (like tumor, infection) involving the hypothalamus and pituitary (especially the front part or the anterior part) these signals from the brain to the thyroid may be interrupted. In such cases, even though the thyroid is functioning normally it does not receive signals from the pituitary to secrete thyroid hormones. This type of hypothyroidism caused by deficient production of TSH by the pituitary is known as secondary hypothyroidism.