Causes of Underactive Thyroid (Hypothyroidism)

Primary Hypothyroidism or Underactive Thyroid can be caused due to several reasons such as follows:

• Iodine deficiency in food is one of the common causes, especially in certain geographic areas of the world.
• Inflammation of the thyroid gland which causes damage or death of the cells of the thyroid leading to insufficient production of the hormones is a major cause of Primary Hypothyroidism. The main cause of failure of the thyroid gland in this manner is Auto-immune Thyroiditis also known as Hashimoto's Thyroiditis. In this disorder, the patient's own immune system causes an inflammation of the gland.
• Post-thyroidectomy: Surgical removal of the thyroid gland or a nodule for treating hyperthyroidism often makes the patient land up with Hypothyroidism after a few years.
• Post-radiotherapy: Radiotherapy is given for hyperthyroidism in order to destroy the hyperactivity of the gland. If a larger number of cells are destroyed by the radioactive iodine, the patient may have insufficient hormone production by the remaining cells and this may cause hypothyroidism.

Secondary hypothyroidism is commonly caused by the following:

• Failure of the pituitary gland to secrete Thyroid Stimulating Hormone (TSH). This may be caused due to:
  • Tumor in the pituitary region
  • Infiltration of the pituitary by inflammatory cells from the immune system
  • Infiltration of the pituitary by foreign substances (such as iron in Hemochromatosis)
• Hypothalamic dysfunction
  Risk factors for secondary hypothyroidism include being over 50 years old, being female, and having a history of pituitary or hypothalamic dysfunction.