Health Education

What is a TSH test and what do your results mean?

Avatar By | January 20, 2020
Medically reviewed by Anis Rehman, MD

The thyroid-stimulating hormone (TSH) test evaluates thyroid function and thyroid hormone levels. It’s produced by the pituitary gland, which tells your thyroid, located in the lower front of your neck, to make and release hormones that control our body temperature, metabolism, and keep our brains, hearts, and other organs working properly. 

The American Thyroid Association (ATA) recommends that adults begin TSH testing at the age of 35 and repeat tests every five years if results are in the normal range. Your doctor may also order a TSH test if you’re experiencing symptoms of an underactive or overactive thyroid. Symptoms of hypothyroidism (an underactive thyroid) can include fatigue, brain fog, extremely dry skin, hair loss, and feeling cold. Symptoms of hyperthyroidism (an overactive thyroid) can include sweating, a fast heart rate, muscle weakness, anxiety, and irritability. In either case, you—or your healthcare provider—might find that your thyroid is enlarged, which is called a goiter, or thyroid nodules, which are small lumps on the thyroid gland.

What does the TSH test involve? 

The TSH blood test is often administered as part of a panel of blood tests in conjunction with an annual health exam. The test determines how well your thyroid is working by measuring the amount of thyroid-stimulating hormone in your blood. It’s a non-fasting blood test, meaning you don’t have to do anything special to prepare. 

Physicians should take a TSH blood sample for thyroid patients every four to eight weeks after their initial diagnosis. The ATA recommends TSH testing every six to 12 months while you’re on a stable medication dose and more often if your dose has changed. 

What should your TSH level be?

Most laboratories classify normal TSH levels as between 0.4 and 4.5 milliunits per liter (mU/L), according to the American Thyroid Association (ATA). A normal TSH level in most cases, usually indicates that you don’t have a thyroid problem.

The ATA reports that nearly 20 million Americans have some form of thyroid disease. Yet more than 60% of those with a thyroid disorder are unaware of their medical condition. 

What does it mean when your TSH is low?

In most cases, TSH levels lower than 0.4 mU/L indicate hyperthyroidism, also known as overactive thyroid, meaning, your body produces too much thyroid hormone. Graves’ disease is an autoimmune disorder that sometimes causes hyperthyroidism.

What happens when your TSH is high?

Generally, TSH levels higher than 4.5 mU/L indicate hypothyroidism, or an underactive thyroid, meaning your body isn’t making enough thyroid hormone. Hashimoto’s thyroiditis is an autoimmune disorder that sometimes causes hypothyroidism.

Can you still have a problem if your TSH is normal?

“The TSH test is a highly sensitive indicator of your overall thyroid health,” says Brittany Henderson, MD, a board-certified endocrinologist at Charleston Thyroid Center in Mount Pleasant, South Carolina. “Interpretation can be difficult because there is a wide range of ‘normal’ on the TSH scale.”

Medications and supplements you’re currently taking can affect the results of your TSH test. One example is biotin, which can falsely show low TSH levels.

What’s considered normal is influenced by several factors:

  • Age
  • Pregnancy
  • Ethnicity

Research has shown that levels of TSH tend to increase with age, decrease with pregnancy, and vary by ethnicity. 

Age

One study found that 97.5% of patients up to the age of 55 years had TSH values less than 4.0 mU/L. Above that age the upper value for the 97.5th percentile gradually rose, reaching about 4.75 mU/L between 75 and 85 years and 5.0 mU/L in patients between 85 and 90. The study went on to say that slightly high TSH levels in older adults don’t necessarily require treatment.

TSH levels can also vary for children and the range can change according to age. It’s recommended that parents check with a pediatric endocrinologist to determine their child’s optimal TSH range.

Pregnancy

In pregnant women in their first trimester, the American Thyroid Association recommends TSH levels are maintained based on the status of thyroid autoimmunity. Usually, TSH is maintained between 0.2-<2.5 mU/L or the upper limit of TSH ie, 4.5 mU/L based on autoimmunity. TSH is recommended to be maintained between 0.3-3 mU/L in the remaining trimesters.

Ethnicity

Dr. Henderson, who co-authored the book, What You Must Know About Hashimoto’s Disease, notes that TSH results can also vary according to ethnicity. “TSH levels are traditionally higher in the white population, lower in the black population, and somewhere in the middle for the Hispanic population,” she explains.

She says patients might have TSH results in the ‘normal’ range, while still experiencing active symptoms (such as weight loss or gain, hair loss, fatigue, anxiety and more). TSH levels indicate if your thyroid has a problem, but they don’t indicate what’s causing the problem. To find that out, your healthcare provider may order additional tests.

When you might need additional tests

In these cases, doctors may choose to order a comprehensive panel of thyroid function tests to measure thyroxine such as Free T4, Total T4, Total T3, and thyroid antibodies, in addition to the TSH. A critical part of obtaining the best possible TSH test results and thyroid treatment involves ensuring you’re receiving the right diagnostic tests. 

T4 test

A Free T4 is a non-fasting blood test that measures levels of T4, a thyroid hormone. A normal level of Free T4 ranges between 0.8 to 1.5 micrograms per deciliter (ng/dL). It can vary depending on the laboratory assay used. Total T4 is not normally done except in pregnancy when it is more accurate than Free T4 test. A normal Total T4 range is 5-12 ug/dl. Low levels of T4 are seen with hypothyroidism, while high levels are seen with hyperthyroidism. 

T3 test

A Total T3 non-fasting blood test is sometimes recommended as a way of diagnosing hyperthyroidism and determining the severity of the disease. Normal Total T3 results are measured as ranging from 100 to 200 nanograms per deciliter (ng/dl). Free T3 test is not recommended due to its laboratory assay inconsistency. Reverse T3 has a limited role in diagnosing thyroid problems and is routinely not recommended. 

When do you need treatment? 

“Many hypothyroid patients feel at their best when they’re on medication and their thyroid levels are in the ‘optimal’ TSH range of 0.5-2.00 (with a lab scale of 0.45-4.5),” Dr. Henderson says. “Since lab assays and ranges vary, the optimized levels for a patient’s specific lab can also vary.” For elderly patients, the TSH target is in the 4.0 – 6.0 range. 

Most doctors commonly treat their hypothyroid patients with a daily dose of levothyroxine, a man-made version of the T4 thyroid hormone made by the body, some doctors recommend using both T4 and T3 as the treatment option.  

RELATED: 5 things that can mess with your thyroid medication

“With thyroid medication, the dosing has to be just right,” Dr. Henderson says. “Too much or too little of the medication can lead to side effects that affect a patient’s health.”

In patients who are diagnosed as hyperthyroid, treatment can include antithyroid medications such as methimazole, that works to control the overactive thyroid. Some patients are given radioactive iodine, which is given in pill form to gradually and safely shrink the thyroid.

If you are experiencing signs of an under- or overactive thyroid, seek professional medical advice from your doctor or a specialist in endocrinology about these tests and treatments.