The main way to determine if you have a thyroid disorder is a TSH blood test – here’s what to expect

• A TSH test measures blood levels of the thyroid stimulating hormone to diagnose a thyroid disorder.
• Your doctor will order a TSH test if you exhibit thyroid disorder symptoms like hair loss or fatigue.
• High TSH levels could be a sign of hypothyroidism while low TSH levels may signal hyperthyroidism.
• Visit Insider's Health Reference library for more advice.

Thyroid disorders, which affect about 20 million Americans, can be diagnosed with a TSH test, which measures levels of what is called the thyroid stimulating hormone (TSH) in your blood.

TSH is a hormone produced in the brain's pituitary gland that supports thyroid function, says Juan Jaume, MD, an endocrinologist and professor at the University of Toledo College of Medicine and Life Sciences.

Thyroid function is important because it helps control your metabolism as well as basic growth and development. So, if you have a thyroid disorder, it can lead to all sorts of issues like weight gain, fatigue, hair loss, and much more.

Here's a look at how the TSH test works and how to interpret its results.

What is a TSH test?

A TSH test is designed to identify abnormal levels of TSH in your blood, which may indicate a thyroid disorder, says Aleem Kanji, MD, an endocrinologist with Ethos Endocrinology, a private practice.

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Typically, your doctor will order a TSH test if you're exhibiting key symptoms of thyroid dysfunction, like excessive hair loss or changes in weight.

The test will involve a blood draw, usually first thing in the morning, and results take around 24 hours.

Interpreting TSH levels

Generally, normal TSH levels are between 0.5 to 5.0 mIU/L. However, this may vary for pregnant people as TSH levels naturally change as pregnancy progresses, Jaume says.

TSH levels below 0.5 mIU/L or greater than 5.0 mIU/L may indicate different types of thyroid disorders that often require additional tests to diagnose. Here's a look at what abnormal TSH levels can mean.

High TSH levels

A high TSH level is anything greater than 5.0 mIU/L and may suggest an underactive thyroid, aka hypothyroidism.

"A significantly elevated TSH isn't dangerous in itself," Kanji says. "However, the significant elevation indicates a concerning or potentially dangerous thyroid disorder."

If you do have hypothyroidism, there are a few different things that could be causing it, but the most common is an autoimmune disorder known as Hashimoto's disease.

To confirm hypothyroidism, a doctor will typically perform a follow-up test that measures your levels of T4, the main form of thyroid hormone circulating in the blood. A high TSH level and a low T4 level may indicate you have an underactive thyroid.

Low TSH levels

A low TSH is anything less than 0.5 mIU/L and typically occurs when thyroid hormone levels in the blood are too high, Kanji says.

That's because, as the brain detects high levels of thyroid hormones, it produces less TSH to stop the thyroid glands from producing more thyroid hormone.

As a result, a low TSH level suggests hyperthyroidism, or an overactive thyroid. There can be a few different causes of hyperthyroidism, but one of the most common is an autoimmune disorder known as Graves' disease.

If your TSH levels are low, your doctor will likely order another test to evaluate your levels of T4 or T3 - another type of thyroid hormone. An elevated T4 or T3 along with a low TSH indicates hyperthyroidism, Kanji says.

How to treat thyroid dysfunction

You can't increase or decrease TSH levels on your own. Therefore, whether you have hypothyroidism or hyperthyroidism, treatment will probably require some form of medical aid.

Treating hypothyroidism

Treatment options for hypothyroidism most often include medication that restores adequate hormone levels.

"There's nothing you can do on your own to lower your TSH levels," Jaume says. "Thyroid autoimmune hypothyroidism requires thyroid hormone replacement."

Treating hyperthyroidism

If your doctor finds other markers of hyperthyroidism, they may suggest different treatment options, including:

• Anti-thyroid medication
• Beta-blockers
• Radioactive iodine
• Surgery to remove the thyroid

Insider's takeaway

A TSH test is one of the first tests used to determine whether or not someone has a thyroid disorder. This test measures the levels of TSH produced by the pituitary gland in your brain.

A high TSH level could indicate hypothyroidism, or an underactive thyroid, while a low TSH level could indicate hyperthyroidism, or an overactive thyroid. If your TSH levels are abnormal, your doctor will likely order more tests to determine the root cause.

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