Hypothyroidism can develop due to a number of reasons and these may vary from patient to patient. Some of the common causes have been listed here:

Severe iodine deficiency:

A diet that is persistently low in iodine (such as that seen in certain geographical areas of the world like India, Chile, remote mountainous regions of Himalayas, Andes, etc.) can lead to Hypothyroidism.

Hashimoto's Thyroiditis:

This is an inherited auto-immune condition which causes the body's immune system to inappropriately attack the thyroid gland. It is 5-10 times more common in women as compared to men. The patient usually presents with am enlarged thyroid gland (goiter) which has diminished ability to produce the hormones T3 and T4. Hashimoto's Thyroiditis is the commonest cause of Hypothyroidism in USA.

Pregnancy:

In some women, there is production of antibodies to the thyroid gland during or after pregnancy leading to hypothyroidism. This can lead to complications during the pregnancy or childbirth and can also have effects on the growing fetus.

Lymphocytic thyroiditis:

Inflammation of the thyroid gland caused by a particular type of blood cell - Lymphocyte - is called Lymphocytic Thyroiditis. This condition is particularly common after pregnancy and the patient usually experiences a phase during which there is excessive production of thyroid hormones from the thyroid gland followed by reduced production of the same. The condition usually resolves within 6 months in most cases though in some cases the patient may continue to remain hypothyroid.

Radioactive iodine treatment or surgery:

Treatment with radioactive iodine or surgical removal of the thyroid gland (while being treated for hyperthyroidism) may lead to reduced production of the thyroid hormones.

Pituitary or Hypothalamic disease:

When the thyroid gland does not receive adequate stimulus for the production of the hormones, it cannot function normally. This is what happens when the pituitary or the hypothalamus are afflicted by disease or injury.

Certain drugs:

Some medicines used for treating hyperthyroidism such as methimazole, propylthiouracil; lithium containing psychiatric medications, etc. can lead to hypothyroidism.

Congenital absence of thyroid gland or a defective thyroid gland:

This can lead to congenital hypothyroidism which needs to be detected in time to prevent complications in the infant.