Low TSH: what it means

What TSH measures

TSH (thyroid stimulating hormone) comes from the pituitary gland in the brain and tells the thyroid how much hormone to make. When there is more thyroid hormone than the body needs, the pituitary lowers TSH. So a low TSH often goes with an overactive thyroid (hyperthyroidism). Free T4 and free T3 show how much active hormone is actually in your blood. Some reports give these in pmol/L: multiply free T4 in ng/dL by 12.9, or free T3 in pg/mL by 1.54, to convert.

Common causes

Graves' disease, an autoimmune condition, is the most common cause of an overactive thyroid. Thyroid nodules that make their own hormone are more common in older adults and in areas with low iodine. In early pregnancy, TSH often dips a little because of pregnancy hormones, which is usually normal. Large amounts of iodine, for example from the heart medicine amiodarone or the contrast dye used in some scans, can occasionally trigger it as well. Your doctor will ask about any recent pregnancy, illness or new medicines.

• Graves' disease
• One or more overactive thyroid nodules
• Thyroiditis, a short-lived inflammation of the thyroid, sometimes after a virus or childbirth
• Too high a dose of thyroid hormone tablets
• Early pregnancy, steroid medicines or recent serious illness

Reading TSH with free T4 and free T3

A low TSH with high free T4 (above 1.8 ng/dL) or high free T3 (above 4.2 pg/mL) fits overt hyperthyroidism. A low TSH with normal free T4 and T3 is called subclinical hyperthyroidism, and it is often watched and retested. Rarely, a low TSH with low free T4 means the pituitary itself is not working well. Symptoms of an overactive thyroid can include a fast or uneven heartbeat, shaking hands, sweating, weight loss, anxiety, poor sleep, loose stools and a swelling in the neck.

What usually happens next

Your doctor will often repeat the test along with free T4 and free T3, and may check thyroid antibodies (such as TSH receptor antibodies) or arrange a thyroid scan or ultrasound to find the cause. If you take thyroid tablets, your dose may be reviewed. Treatment for an overactive thyroid depends on the cause and can include tablets that calm the thyroid, radioactive iodine or, less often, surgery. Your doctor may also suggest a medicine to slow a fast heartbeat while tests are done. A very low TSH over a long time can affect heart rhythm and bone strength, so follow-up matters.

Questions to ask your doctor

• Are my free T4 and free T3 high, normal or low?
• What is the most likely cause of my low TSH?
• Do I need antibody tests, an ultrasound or a thyroid scan?
• If I take thyroid tablets, should my dose be reviewed?
• Should my heart rhythm or bone health be checked?