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Thyroid stimulating hormone (TSH) is produced in the pituitary gland in order to regulate the production of thyroid hormones thyroxine (T4) and triiodothyronine (T3) by the thyroid gland. If thyroid hormones in the blood are low, then more TSH is produced to stimulate the thyroid gland to produce more of them. If thyroid hormone levels are high, then the pituitary produces less TSH to slow the production of thyroid hormones.

If TSH is too high or too low, it normally signifies that there is a problem with the thyroid gland which is causing it to under or over produce thyroid hormones. Sometimes a disorder of the pituitary gland can also cause abnormal TSH levels.

What might a low result mean?

Low TSH means that your thyroid gland is over-producing thyroid hormones and you may have an overactive thyroid (hyperthyroidism). An overactive thyroid means that your body's metabolism is speeded up which can cause many symptoms, including feeling too hot, losing weight, feeling nervous and jittery and having a rapid heart beat. An overactive thyroid also is associated with health risks like atrial fibrillation and osteoporosis (weak bones).

Low TSH can also occur in people who have a diagnosed thyroid condition and are taking replacement thyroid hormones. It can mean that their medication needs to be reduced, especially if it is accompanied by symptoms of an overactive thyroid.

Low TSH that is accompanied by low levels of T3 and T4 can be a sign of an underlying pituitary problem.

What might a high result mean?

High TSH means that your thyroid gland is under-producing thyroid hormones and you may have an underactive thyroid (hypothyroidism). An underactive thyroid means that your body isn't producing sufficient thyroid hormones to keep your metabolism running at a healthy level. Symptoms are numerous and typically include putting on weight, finding it difficult to lose weight, feeling cold when people around you are warm, feeling very tired and having no energy for day-to-day activities and having dry skin and hair. Hypothyroidism carries long term health risks such as elevated cholesterol and a higher risk of heart disease.