Low levels of thyroid hormone interfere with metabolism.  Sometimes the thyroid gland does not make enough hormone, especially in older women.   Symptoms of hypothyroidism can include fatigue, a puffy face, weight gain, irregular periods, depression, muscle aches, weakness, memory dysfunction, or dry skin.

Hypothyroidism if diagnosed by blood tests.  People who have a high TSH and a low T4 are hypothyroid. Sometimes people may also have a low T4 and an inappropriately low TSH.
Treatment of hypothyroidism is usually done with a thyroid hormone supplement.