Hypothyroidism is a condition when the thyroid produces too little thyroid hormone and may cause the body’s metabolism to slow down.
Hypothyroidism commonly is due to a problem in the thyroid gland. However, hypothyroidism can develop following treatment for hyperthyroidism (overactive thyroid) such as radioactive iodine therapy or following surgery to remove all the thyroid gland.
There is a simple blood test call thyroid function test which can diagnose if the thyroid is underactive.
Treatment for hypothyroidism involves taking thyroid hormone tablets daily. The response to the thyroid tablet is monitored with a blood test (thyroid function test).
Useful link
http://www.thyroid.org/thyroid-information/