What Is a TSH Test?

A TSH test is done to find out if your thyroid gland is working the way it should. It can tell you if it’s overactive (hyperthyroidism) or underactive (hypothyroidism). The test can also detect a thyroid disorder before you have any symptoms. If untreated, a thyroid disorder can cause health problems.

TSH stands for “thyroid stimulating hormone” and the test measures how much of this hormone is in your blood. TSH is produced by the pituitary gland in your brain. This gland tells your thyroid to make and release the thyroid hormones into your blood.

The Test

The TSH test involves simply drawing some blood from your body. The blood will then be analyzed in a lab. This test can be performed at any time during the day. No preparation is needed (such as overnight fasting). You shouldn’t feel any pain beyond a small prick from the needle in your arm. You may have some slight bruising.

In general, there is no need to stop taking your medicine(s) before having your TSH level checked. However, it is important to let the doctor know what medications you are taking as some drugs can affect thyroid function. For example, thyroid function must be monitored if you are taking lithium. While taking lithium, there is a high chance that your thyroid might stop functioning correctly. It's recommended that you have a TSH level test before starting this medicine. If your levels are normal, then you can have your levels checked every 6 to 12 months, as recommended by your doctor. If your thyroid function becomes abnormal, you should be treated.

High Levels of TSH

TSH levels typically fall between 0.4 and 4.0 milliunits per liter (mU/L), according to the American Thyroid Association. Ranges between laboratories will vary with the upper limit being between 4 to 5. If your level is higher than this, chances are you have an underactive thyroid.

In general, T3 and T4 levels increase in pregnancy and TSH levels decrease.

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Low Levels of TSH

It's also possible that the test reading comes back showing lower than normal levels of TSH and an overactive thyroid. This could be caused by:

  • Graves’ disease (your body’s immune system attacks the thyroid)
  • Too much iodine in your body
  • Too much thyroid hormone medication
  • Too much of a natural supplement that contains the thyroid hormone

If you're on medications like steroids, dopamine, or opioid painkillers (like morphine), you could get a lower-than-normal reading. Taking biotin (B vitamin supplements) also can falsely give lower TSH levels.

The TSH test usually isn’t the only one used to diagnose thyroid disorders. Other tests, like the free T3, the free T4, the reverse T3, and the anti-TPO antibody, are often used too when determining whether you need thyroid treatment or not.

Treatment

Treatment for an underactive thyroid usually involves taking a synthetic thyroid hormone by pill daily. This medication will get your hormone levels back to normal, and you may begin to feel less tired and lose weight.

To make sure you're getting the right dosage of medication, your doctor will check your TSH levels after 2 or 3 months. Once she is sure you are on the correct dosage, she will continue to check your TSH level each year to see whether it is normal.

If your thyroid is overactive, there are several options:

  • Radioactive iodine to slow down your thyroid
  • Anti-thyroid medications to prevent it from overproducing hormones
  • Beta blockers to reduce a rapid heart rate caused by high thyroid levels
  • Surgery to remove the thyroid (this is less common)

Your doctor may also regularly check your TSH levels if you have an overactive thyroid.

WebMD Medical Reference Reviewed by Nayana Ambardekar, MD on March 24, 2019

Sources

SOURCES:

Mayo Clinic: "Hypothyroidism (underactive thyroid)."
American Thyroid Association: "Thyroid Function Tests."
Medscape: "Thyroid-Stimulating Hormone."
National Academy of Hypothyroidism: "How Accurate is TSH Testing?"
UptoDate.com: "Lithium and the Thyroid."
American Thyroid Association. Clinical Thyroidology for the Public: "Thyroid Function Tests."
MedicalNewsToday: Jayne Leonard, "What do TSH levels mean?" 
Medscape: "Guidelines for Thyroid Disease in Pregnancy: Key Points."
 

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