The thyroid is a butterfly-shaped gland located in your throat beneath the larynx. It produces T3 and T4 thyroid hormones, which regulate metabolism and growth. Hypothyroidism results when the thyroid gland doesn’t create enough of these hormones.

Iodine, which enters your body through the food you eat, is a main component of both hormones. So a diet that is low in iodine may also be the cause of an underactive thyroid.

When your body produces less than an optimal level of thyroid hormones, the functioning of all your body processes, known collectively as your metabolism, slows down, from the top of your head to the tip of your toes. Weight gain is common as food is metabolized slower. In addition, your energy levels decrease, your cognitive abilities and mood decline, your body temperature lowers, and your pulse weakens.

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There are several blood tests used to diagnose hypothyroidism. The thyroid stimulating hormone (TSH) test is the most accurate and commonly performed because TSH controls thyroid hormone production.

TSH is created by the pituitary gland in your brain. When the thyroid isn’t making enough thyroid hormone, the pituitary discharges more TSH than is needed as it attempts to balance the system. So a TSH level higher than normal indicates hypothyroidism.

If a blood test shows abnormally high TSH levels, your doctor may order additional tests to identify the root cause of the problem.

Free T4 and free T3 tests could shed more light on the issue. Most T4 is bound to a protein in the blood and doesn’t affect body function. However, unbound or free T4, which makes up less than 1 percent of all T4, does affect body functions. As with T4, less than 1 percent of T3 is unbound, but that small amount affects metabolism more than free T4. A lower than optimal level of free T4 or T3 helps pinpoint the cause of hypothyroidism.