If your doctor wants to determine whether your thyroid is healthy, one of the blood tests you may need will look at whether you have normal levels of thyroid stimulating hormone (TSH). This can suggest if you need further tests or treatment.
How the Thyroid Works
The thyroid is a butterfly-shaped gland located in the middle of your lower neck. Its main job is to produce hormones that, among other important functions, control your body's metabolism — meaning the pace at which your cells carry out the tasks they need to do in order for you to function, according to the Cleveland Clinic.
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The thyroid produces thyroxine (T4) and triiodothyronine (T3). "The important difference is that T3 is the active form of the hormone, while T4 has very little activity," says Elizabeth A. McAninch, MD, an assistant professor in the Department of Internal Medicine at Rush Medical College in Chicago. "Many tissues in your body can convert T4 to T3 when they need it."
Thyroid hormones must be properly balanced so your body doesn't use energy too quickly (hyperthyroidism) or too slowly (hypothyroidism), Dr. McAninch says. Another hormone — thyroid-stimulating hormone (TSH) — can provide information about whether you have too much or too little thyroid hormone. In fact, the TSH test can alert you and your doctor to thyroid disorders even before you develop any symptoms.
What Is TSH?
TSH is not made by the thyroid. It's manufactured in the pituitary gland — a pea-size gland located just beneath the base of the brain, behind the bridge of your nose, Dr. McAninch says.
"The pituitary makes TSH and secretes it into the bloodstream, where it circulates and encounters the thyroid gland," she says. "Once the thyroid senses TSH, it starts making hormones based on those instructions. If the TSH is high, the thyroid will respond by making more thyroid hormones, and if the TSH is low, the thyroid will produce less thyroid hormone."
But it's not all one-way traffic, she points out. "If the thyroid isn't producing enough hormones, the pituitary will increase TSH, which can, in turn, stimulate the thyroid to produce more," Dr. McAninch says. "On the other hand, if the thyroid is producing too much hormone, the pituitary reduces TSH so thyroid produces less hormone."
What High TSH Means
High TSH may be a clue your thyroid hormone levels are too low — in which case you likely have hypothyroidism, Dr. McAninch says. In other words, high TSH but low thyroid hormones indicate your thyroid is not working well and can't make enough thyroid hormone, even though the brain is asking it to make more. Symptoms of hypothyroidism can include fatigue, lethargy, forgetfulness, weight gain, foggy brain, constipation, dry skin, brittle hair/nails and feeling cold.
Hypothyroidism can be treated with levothyroxine, a thyroid replacement hormone that is the synthetic form of T4 (brand names include Levothroid, Levoxyl and Synthroid), according to the Mayo Clinic.
"Levothyroxine is the standard of care for hypothyroidism treatment, and most people do well, can achieve normal, healthy thyroid hormone levels and have healthy pregnancies," Dr. McAninch says.
TSH is often used to monitor thyroid hormone replacement if you have hypothyroidism, according to the American Thyroid Association. High TSH suggests your thyroxine dose should be increased, while low TSH suggests your thyroid hormone dose is too high and should be reduced. For most people on thyroxine replacement, the goal TSH level is between 0.5 to 2.5 milliunits per liter (mU/L).
What Low TSH Means
Low TSH usually suggests your thyroid is producing too much thyroid hormone. Your symptoms might include feeling hot, feeling anxious, weight loss, tremors, frequent bowel movements, unusually soft skin, menstrual irregularities and a high metabolic rate, Dr. McAninch says.
How this is treated depends on the cause, she says. One cause may be Graves' disease, an autoimmune condition that causes the thyroid to make too much hormone.
Hyperthyroidism can be treated with medication, but according to the American Thyroid Association, some people may need surgery to remove all or most of the thyroid gland or radioactive iodine to damage overactive thyroid tissue.
- Elizabeth A. McAninch, MD, assistant professor, Department of Internal Medicine, Rush Medical College, Chicago, Illinois
- Cleveland Clinic: “Thyroid Disease”
- Mayo Clinic: “Levothyroxine (Oral Route)”
- American Thyroid Association: “Is the TSH (Thyroid Stimulating Hormone) a Good Way to Titrate My Thyroid Hormone Therapy?”
- American Thyroid Association: “Hyperthyroidism (Overactive)”
Is this an emergency? If you are experiencing serious medical symptoms, please see the National Library of Medicine’s list of signs you need emergency medical attention or call 911.