Here's What the Numbers on Your Thyroid Test Mean

More than 12 percent of people in the U.S. will grapple with a thyroid condition at some point, and right now, about 20 million Americans have thyroid disease, according to the American Thyroid Association (ATA).

But thyroid disease can be sneaky, with symptoms — like fatigue and constipation — that are often easy to confuse for other problems. Because of that, the ATA points out, as much as 60 percent of those with a thyroid condition don't know they have it.

The thyroid is a butterfly-shaped gland at the base of your neck. And while it may be small, it plays a big role in your overall health: It controls your metabolism, according to the Hormone Health Network, which is run by the Endocrine Society. That's why a thyroid that's overactive (hyperthyroid) or underactive (hypothyroid) is associated with symptoms like heat or cold intolerance, unintended weight loss or gain, sleep problems, gastrointestinal distress and more.

If you're having symptoms that can be chalked up to a thyroid issue or have found a mass or lump in your neck at your thyroid gland, then your doctor (or you!) may request a thyroid test, says internal medicine physician Kristen Harvey, MD, ZoomCare Daily Care Practice Lead in Colorado and Idaho. Thyroid levels are measured by a standard blood test.

After your blood test, it's natural to want to understand your results on your own or at least know what you're looking at, especially if you've gotten your results through a health portal before your doctor has looked at them. We'll break it down for you here, but you'll want to have a conversation with your doctor as well.

"There is no cookbook approach for thyroid lab interpretation, but that's what will happen if you try to interpret your lab on your own before talking to your doctor," says Quang Nguyen, DO, medical director for Las Vegas Endocrinology and contracted specialist at AristaMD.

For example, you may interpret a specific number as evidence of thyroid dysfunction when in reality it was a normal variation.

With that said, here's some insight about what you'll find on a thyroid test, but remember to discuss your results and ask your doctor all the questions you need in order to get the best care possible.

Thyroid blood tests most commonly include the following:

The first screening test you'll receive is TSH, says Dr. Nguyen. "This test is the most sensitive and specific test to assess thyroid disease," he says.

TSH stands for thyroid stimulating hormone, which is the thyroid-regulating hormone made in a pea-sized gland at the base of the brain called the pituitary. TSH levels will tell you how well the thyroid is working, per the U.S. National Library of Medicine (NLM).

To compensate for too-low (hypothyroid) thyroid levels in the body, the pituitary will ramp up production of TSH, and so you'll see a high TSH number. The opposite is true for high (overactive) levels of thyroid hormones: you'll see a low TSH level.

According to the NLM, normal TSH levels range between 0.5 to 5 µU/mL, but keep in mind that certain medications and supplements can affect your levels, and they tend to fluctuate throughout the day.

If the TSH test is abnormal, your provider will order a free T4 test to confirm the type of thyroid disease (over- or underactive), says Dr. Harvey.

T4 is also called thyroxine, which is a hormone released by the thyroid. You can get a free T4 or bound T4 test, though free T4 is considered a more sensitive — and thus more accurate — test, she says.

According to the Cleveland Clinic, normal T4 levels for adults are as follows:

• ​T4:​ 5.0 – 11.0 ug/dL
• ​Free T4:​ 0.9 - 1.7 ng/dL

T3 is called triiodothyronine, another hormone made by your thyroid. T3 is tricky, as levels can change by the hour or minute, says Dr. Nguyen. It is not used to diagnose hypothyroidism (underactive thyroid), but is used in hyperthyroidism (overactive thyroid), he says.

Normal T3 ranges in adults are as follows, per the Cleveland Clinic:

• ​T3:​ 100 - 200 ng/dL
• ​Free T3:​ 2.3 - 4.1 pg/mL

Your doctor will interpret your thyroid lab results, but that won't be the final word.

"Evaluating thyroid dysfunction requires looking at the whole person. The lab panel is just part of the evaluation," says Dr. Nguyen.

Your doctor should also do a deep dive into the symptoms you're experiencing and consider other causes. For example: Many patients who see Dr. Nguyen for a thyroid evaluation actually have sleep apnea.

That said, your doctor may tell you that you have a sluggish thyroid, which is an underactive thyroid, or hypothyroidism. Or you may be told that your thyroid is in overdrive, and you have an overactive thyroid, or hyperthyroidism.

"Once an individual is diagnosed, treatment usually begins in order to manage symptoms because these diseases can be very debilitating," says Dr. Harvey.

For hypothyroidism, you may be given a medication, aka replacement therapy, which — as it sounds — is a synthetic thyroxine (T4) medication that replaces the hormone you're missing, per the American Thyroid Association.

When it comes to hyperthyroidism, this may be treated with antithyroid medications or radioactive iodine, notes the Hormone Health Network.

One cause of thyroid disease is autoimmune disorders, such as Graves' disease, which leads to an overactive thyroid, and Hashimoto's thyroiditis, a type of underactive thyroid.

A marker for thyroid autoimmune conditions is having certain types of thyroid antibodies. Not all thyroid conditions stem from an autoimmune disorder, but "the most common cause of thyroid dysfunction is autoimmune," says Erin Okawa, MD, attending physician at UCLA and contracted specialist at AristaMD.

To completely rule out thyroid disease, you should have a full thyroid panel done, says Dr. Nguyen. That includes TSH, free T4, free T3 and thyroid antibodies.

Lastly, there are things that can interfere with thyroid lab results. Namely: Are you taking biotin supplements? The B vitamin is famous for being in the mix in supplements that promote hair, skin and nail health, but high doses of biotin can lead to "falsely low TSH levels and falsely high free T4 levels," says Dr. Okawa.

If you're taking biotin and have a mildly abnormal test, your doctor may want to recheck your levels. "An endocrinologist is the best person to help interpret these results if they are abnormal, and I encourage readers to seek specialist care if they have multiple abnormal labs," Dr. Okawa says.