Every Important Thyroid Function Test You Need, Explained

Gretchen Lidicker
Medically Reviewed
February 20, 2020

Take a glance at your last round of blood work—did it include a thyroid function test that looked at your thyroid stimulating hormone (TSH)? If it did, chances are that was the extent of the attention your thyroid got. But there are actually a lot of other things a thyroid panel should include.

At Parsley Health, thyroid health is a topic we discuss every single day, and we approach thyroid testing a little differently than most doctors, looking at markers of thyroid health outside of just TSH. Here’s what you need to know about your thyroid, thyroid testing, and how to support your thyroid naturally .

What does the thyroid do?

The thyroid is a gland that sits in the lower-front part of your neck. And while it may be small, it has many big jobs of helping to regulate bodily functions such as energy levels, digestion, body temperature, metabolism, and brain function.

As Rachael Gonzalez, MD , a board-certified integrative and family medicine physician at Parsley Health in Los Angeles, explains it: “The thyroid is a gland at the intersection of a lot of hormonal orchestration.” It is also intricately connected to lifestyle factors, especially our stress levels and cortisol levels . “The hypothalamus and pituitary, which mediate day-to-day stressors, have a downstream effect on glands like the thyroid and ovaries, which are very sensitive to illness, toxic exposures, and stress levels,” explains Dr. Gonzalez.

Knowing this it won’t surprise you to learn that when you put it under stress, the thyroid can start to go haywire. This can cause it to produce too many hormones—which is called “hyperthyroidism” and causes symptoms like anxiety , tremors, and weight loss—or not enough, a condition called “hypothyroidism” that causes symptoms like weight gain, depression, and fatigue .

In fact, according to the American Thyroid Association , about 20 million Americans have some form of thyroid disease—and as much as 60 percent are unaware that they have a condition.

The top thyroid tests you need

If you think you have a thyroid issue, the first step is to test your thyroid hormone levels through a blood test. Most conventional medicine doctors will look at your TSH levels. TSH is a hormone produced in the brain that tells the thyroid gland to make more T3 and T4 thyroid hormones in response to environmental factors. As Dr. Gonzalez explains it: “It’s a way of fine-tuning thyroid output based on the brain, which is trying to protect us from stress and infection.” And while TSH is the foundation for assessing thyroid function, measuring TSH alone isn’t enough to give you a full picture of the status of your thyroid health.

At Parsley Health it’s standard practice to test for TSH as well as the following comprehensive thyroid function tests to help determine not only if the values are out of range but how they compare to a patient’s previous lab results—and how this may be changing over time.

Free T4

T4 is the hormone that’s made in the greatest quantity but according to Dr. Gonzalez, “95% of T4 is bound to a protein , so you have to measure what’s free to know how much is really usable by the body.” T4 can also be converted into T3, which is stronger and more usable by the body.

Total T4

“This measures what’s bound and unbound to proteins in the body,” says Dr. Gonzalez. This test provides complementary information to the free T4 test to help understand the overall picture of your thyroid health, including how your binding proteins may differ.

Free T3

T3 may be the most important thyroid hormone made by the thyroid gland; it helps control body temperature, brain function, heart health, and energy levels. It’s considered stronger than T4 and too high or too low free T3 levels can cause the classic symptoms of hyper- or hypothyroidism.

Reverse T3

“This is the mirror image of T3,” says Dr. Gonzalez. According to her, high T3 levels are the body’s way of giving itself a break. “If you’re running on high cortisol your thyroid will convert T4 to reverse T3 instead of T3. Reverse T3 will occupy the receptor and not allow T3 to bind. “If reverse T3 is high, it’s a sign that the patient has an underlying infection, sleep issues, cortisol issues, or that they may be eating too many carbs.”

Anti-TPO Antibodies

TPO, which stands for thyroid peroxidase, is a thyroid gland enzyme that plays an important role in the production of thyroid hormones. When you have TPO antibodies, it can indicate an autoimmune disorder of the thyroid, such as Hashimoto’s or Graves’ disease.


Thyroglobulin is a protein produced in the thyroid gland; elevated levels of thyroglobulin antibodies can indicate Hashimoto’s, especially if you also have anti-TPO antibodies.

Thyroid health is also intricately connected to certain nutrient deficiencies , such as vitamin D , which is why Parsley Health practitioners will also test for common nutrient deficiencies and then connect the data to any symptoms to get the most accurate and comprehensive picture of the status of your thyroid health.

Why your thyroid hormone levels matter

Getting these thyroid tests done is just the first step—next, you need them interpreted accurately. It shouldn’t be so tricky, but it is. “Large labs like LabCorp and Quest have reference ranges that were developed by large studies that had skewed results,” says Dr. Gonzalez. According to her, further data from the American College of Endocrinology has revealed more updated and more accurate reference ranges. Some doctors, like those at Parsley Health, use the more up to date ranges to evaluate thyroid health, but others may use the lab’s ranges to interpret your test results.

According to Dr. Gonzalez, it’s not just about diagnosing thyroid issues when they’re already severe, it’s about catching them early to prevent them from progressing. “It’s important to capture people with early signs and symptoms of hypothyroidism; left untreated it can lead to menstruation irregularities, infertility, heart disease, and stroke,” she says.

What to do if your thyroid function is off

So what do you do if your thyroid test results reveal over- or underactivity? According to Dr. Gonzalez, “Hyperthyroidism is really blatant, less common, and is usually treated with medication.” And although lifestyle changes can be helpful, the treatment for an over-active thyroid is pretty cut and dried and requires medication or even surgery. Left untreated, an overactive thyroid can lead to serious health issues such as heart and eye issues and brittle bones.

When it comes to hypothyroidism, though, things get a lot more nuanced.

Dr. Gonzalez says she sees a lot of overmedication in people with hypothyroidism. “In fact, many people with a hypothyroid issues don’t actually have a thyroid problem at all,” says Dr. Gonzalez. Oftentimes it’s an HPA axis dysfunction or another underlying health concern that’s affecting the thyroid indirectly. As she puts it: “An underactive thyroid is often the body saying you are breaking yourself down faster than you can repair yourself.”

According to Dr. Gonzalez, an underactive thyroid could also be caused by the following underlying issues:

  1. Underlying autoimmune disease , like celiac disease
  2. Imbalance in gut bacteria
  3. A deficiency in iron, tyrosine, zinc, selenium, B1, B3, B6, vitamin C, or vitamin D
  4. Heavy metal or radiation exposure
  5. Overexercising
  6. Very short windows of eating

The good news is that once you address these concerns, you can heal your thyroid by extension. “We actually see hypothyroidism as an opportunity to address other things that might be going on in the body,” says Dr. Gonzalez.

Typically, the first step is adopting a thyroid-friendly diet. On the list of foods that help support your thyroid are Brazil nuts, oysters, dark leafy greens, and dulse flakes. “We always start with our core diet, which is gluten -free and dairy -free. It’s a good idea to use caffeine and alcohol very mindfully as well,” says Dr. Gonzalez.

Even if you’ve been thyroid medication for years, with a thoughtful provider and the right lifestyle changes, you can likely start to scale back.

Overall, a thyroid issue is a sign that your overall lifestyle could use some TLC. As Dr. Gonzalez explains it: “The thyroid is a recipient of all the inflammation and toxicities you’re exposed to everyday. And when it starts becoming dysfunctional, it’s time to pay attention.”

Gretchen Lidicker

Gretchen Lidicker is a writer, researcher, and author of the book CBD Oil Everyday Secrets: A Lifestyle Guide To Hemp-Derived Health & Wellness. She has a masters degree in physiology and complementary and alternative medicine from Georgetown University and is the former health editor at mindbodygreen. She's been featured in the New York Times, Marie Claire, Forbes, SELF, The Times, Huffington Post, and Travel + Leisure.

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