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What is TSH?

Thyroid stimulating hormone affects everything from your fertility to your appetite. Here's how to know if it is working the way that it should.

Woman having a thyroid exam

Thyroid stimulating hormone (TSH), also known as thyrotropin or thyrotrophin, is a hormone that controls the way other hormones function. Basically, it stimulates the production of two main hormones, T4 (thyroxine) and T3 (triiodothyronine), hence its name: thyroid stimulating hormone. Without TSH, the whole system can’t properly function (more on T3 and T4 below).

What does TSH do in the body?

In a complex and elegant system, your pituitary gland, hypothalamus, and thyroid gland all work together.

  • Here’s how: First, the hypothalamus, which is located in the brain, produces a hormone called thyrotropin-releasing hormone. This hormone controls the pituitary gland, and thus, the thyroid stimulating hormone itself.
  • Next, the pituitary gland, which is located at the base of your skull, makes the thyroid stimulating hormone. After it’s made, it’s released right into the bloodstream, and eventually binds to receptors on the cells of the thyroid gland.
  • The thyroid gland is a very small, butterfly-shaped gland at the bottom of your throat. At this point, everything kicks into gear. After the TSH binds to the cells, the thyroid gland is ‘told’ to produce the right amount of T3 and T4 and to release them into the bloodstream.
  • Although TSH controls the release of T3 and T4, T3 and T4 also have an effect on TSH: “Once your T4 and T3 hormone levels reach a certain limit, they send a negative feedback loop to the pituitary to stop producing TSH,” says  Dr. Carrie Lam, MD, FAAMFM, ABAARM. This loop helps to keep the hormones levels balanced.

How do T3 and T4 work?

Along with thyrotropin-releasing hormone and TSH, T3 and T4 are both vital to the body’s healthy function. These hormones control your metabolism, brain development, bone health, heart rate, digestion, and so much more.

More specifically, T4 is responsible for your mood, metabolism, and core temperature, while T3 is responsible for metabolic function and bone development. In a vital process called deiodination, the body converts T4 into T3, which is the biologically active form of the thyroid hormone.

But why is this complicated conversion even necessary? Get ready for a quick science lesson: According to Dr. Yasmin Akhunji is a board-certified endocrinologist with Paloma Health, “T4 is generally inactive and works as a storage hormone to transport T3 to the proper organs in your body. T4 must be converted to T3 before your body can use it for energy in the cells, which mostly happens in the liver.”

Why can’t T4 just do T3’s job? “T4 is both bound and unbound (known as free T4) in the body. Free T4 is not bonded to protein in the blood, allowing it to enter the body tissues that need to use it. However, most of the T4 in the bloodstream is bonded to protein, preventing it from entering these tissues,” says Akhunji. For this reason, T3 is necessary.

T3 is sort of similar: “T3 is also both bound and unbound (free T3). Nearly all of the T3 found in the blood is bound to protein.”

In short, when one hormone is off, it can throw all the others out of whack.

Too much or too little TSH

When your hormone levels are off, you could experience any number of symptoms — although not everyone with a thyroid condition notices symptoms, especially at first. Here’s what you’ll want to watch out for...

Hyperthyroidism (too much thyroid hormone) symptoms may include

  • Rapid heart rate
  • Anxiety
  • Shaky hands
  • Weight loss
  • Hair loss
  • Bulging eyes
  • Sleeping issues

Hypothyroidism (too little thyroid hormone) symptoms may include:

  • Fatigue and sluggishness
  • Weight gain
  • Digestive issues, including constipation
  • Hair loss
  • Dry skin
  • Slow heart rate

If your body is making too much TSH, this may indicate that your thyroid gland isn’t making enough T3 or T4. When TSH is released into the bloodstream and the thyroid gland doesn’t respond properly by making enough T3 or T4, the TSH floods your system.

On the other hand, if your TSH levels are far too low, this might indicate that your thyroid gland is making too much thyroid hormone. When this happens, your TSH hormone may then be suppressed.

This complex balancing act is necessary for healthy endocrine system function — and yet even the smallest imbalance can lead to health concerns worth speaking with a health care provider about, especially if you are pregnant, as thyroid stimulating hormone is responsible for the healthy development of a fetus.

It is important to note that other conditions can lead to imbalance TSH levels, including rare genetic conditions or problems with the pituitary gland.

Testing TSH

If you don’t suspect a thyroid issue, including a TSH test in your routine bloodwork can help clue you into any potential problems.

But if you do suspect hyperthyroidism (high thyroid hormone levels) or hypothyroidism (low thyroid hormones levels), you’ll want to test your TSH. This can be done with a blood test. However, many healthcare practitioners agree that TSH testing should be seen as a bare minimum.

That’s because if your TSH levels are off, a single TSH blood test (which is standard practice) won’t explain why your levels are off. At this point, your healthcare provider will likely order additional tests to fill out the picture of your health.

Additional thyroid hormone tests include

  • A T4 hormone test, usually to determine hypothyroidism
  • A T3 hormone test, usually to determine hyperthyroidism
  • Tests to determine the presence of autoimmune antibodies, which could indicate Graves’ disease or Hashimoto’s disease, both of which are autoimmune diseases that cause hyperthyroidism and hypothyroidism, respectively.
  • Free T3 or T4 tests

In fact, you should ask for additional testing if you do suspect a thyroid issue — even if your TSH levels are normal. A normal TSH could bely any number of issues. For example, your body may have trouble converting T4 into T3 even when TSH is normal.

Dr. Akhuni says, “Testing for TSH alone may not give the complete picture of a patient's thyroid health. For instance, testing for TSH alone may not show problems like your body not being able to use the available thyroid hormone.”

The best bet? “Testing for free T3 in addition to TSH and free T4 can help inform how well your body is converting thyroid hormone to the active form to be used by the body. Each of these thyroid tests is just a glimpse into what's happening in the body, but when looked at all together, they can inform next steps and a personalized treatment plan,” Dr. Akhunji says.

TSH testing after thyroid treatment

Although a TSH test may not reveal the whole picture if you’re unsure of whether you have a condition or not, it can be helpful if you’ve been diagnosed and already started treatment.

According to the American Thyroid Association, testing your TSH levels is a great way to monitor your body’s response to thyroid hormone replacement. For example, if you’re receiving synthetic thyroxine due to hypothyroidism, a TSH test can tell you if it’s working.

Thyroid hormone levels aren’t very intuitive — and they can be hard to understand. While every person is different, often high TSH levels may indicate an underactive thyroid (meaning medication should be increased) while low TSH levels can mean a thyroid is overactive or that medication should be decreased.

Low TSH levels could also stem from an issue with the pituitary gland, rendering it unable to make enough TSH. This is called secondary hypothyroidism.

You’ll want to test your TSH every six to 12 months if you’ve been prescribed medication. if  your dosages are changed, you’ll want to test it more frequently — usually six to eight weeks after medication changes. You’ll need additional testing if you’re on hormone medication while pregnant as well.

Your thyroid system is delicate, and many things can influence it. From autoimmune diseases and inflammation to genetic disorders and nodules, there are many possible root causes behind a thyroid condition. Beyond lab work, getting an ultrasound or thyroid biopsy may also be a worthy option.

What are normal TSH levels?

A normal reference range for TSH levels is somewhere between 0.5 to 5.0 milli-international units per liter (mIU/L), but it’s important to note that different labs — and different healthcare providers — may not agree on that range. In fact, “normal” may look different for each patient.

For example, some doctors believe that the normal range should be narrowed down to 0.4 to 2.5, says Dr. Cory Rice, DO.

Generally, if your TSH is below 0.5 mIU/L, it may mean your thyroid gland is overactive, which could lead to hyperthyroidism. Anything over 5.0 mIU/L may point to hypothyroidism.

Luckily, there are plenty of ways to support your thyroid health

Whether you’ve had thyroid surgery or are prescribed medication to balance your hormones, a healthy lifestyle is key if you’re looking to achieve and maintain healthy thyroid levels. You can support your thyroid by:

  • Keeping stress at bay: It’s impossible to entirely get rid of stress, but managing it is crucial. When chronically stressed, the body creates stress hormones — like cortisol — that tax the system. This can cause fatigue and weight gain, which can further affect your hormones by lowering T3 and T4 and affecting the conversion of T4 into T3. Consider journaling, spending time in nature, yoga, meditation, or tweaking your lifestyle and environment. Make stress management a priority.

  • Eating for thyroid health: While no single food or food group (no matter how healthy!) is a magic fix-all, you’ll want to focus on filling your plate with fresh, colorful foods. Think fruits, lean proteins, veggies, fish, beans, and healthy fats. Skip inflammatory foods and anything processed or full of added sugars. And make sure supplements you’re taking don’t contain hidden T3 or T4. If you’re not sure, as the manufacturer if the product or any ingredient within it has been tested for T3 and T4 adulteration.

  • Adding movement to your routine: Getting regular exercise can help balance your hormones. You don’t have to join the gym to get the benefits, though. Walking daily can improve your health, as can gentle yoga, hiking, swimming, or dancing. If you have hyperthyroidism, talk to your doctor before engaging in exercise, since your metabolism may be sped up and exercise may affect you negatively.

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