Endocrine Community
Get answers. Share advice. Learn More

What Is T4 (Thyroxine)?

This powerful hormone affects everything from your energy level to your body temperature. Here’s what to know about it.

with Yasmin Akhunji, MD, and Smita Koppikar, MBBS, DNB

Your thyroid gland controls a myriad of important body functions, including brain function, digestion, metabolism, and bone health. When your thyroid hormones are thrown off balance, you may find yourself feeling “off.” One of those hormones, thyroxine (referred to as T4), is a major player in your endocrine system. It is responsible for your metabolism, mood, and body temperature, among other things. Here’s a look at how T4 works and what happens if you have too much or too little of it.


In This Article: Definition | Forms of T4 | Effects of Too Much or Too Little T4 | Other Conditions That Affect T4 | Outside Variables That Affect T4 | How T4 Is Tested | What Is a Healthy T4 Level? | Treating a T4 Imbalance


Thyroid Hormones: An Overview

T4 is one of two main thyroid hormones, the other being triiodothyronine (referred to as T3). These two hormones are the thyroid gland’s main players; when you are healthy, they work together in balance and harmony.

Both T3 and T4 are controlled by two other hormones. The first is the thyrotropin-releasing hormone. This is made in the hypothalamus, which is located in the brain. When the hypothalamus releases this hormone, it sends a chemical “message” to the pituitary gland, which is at the base of your skull. The thyrotropin-releasing hormone prompts the creation of thyroid stimulating hormone, also referred to as TSH.

Medical image of the thyroid hormone T4 (thyroxine)T4, or thyroxine, is released by the thyroid gland. Image credit: iStock


When TSH is released into the bloodstream, it travels from the pituitary gland to the thyroid, which is a butterfly-shaped gland located at the base of your neck. TSH stimulates — hence the name — the thyroid gland to produce thyroid hormones. It knows whether your body needs a little more or less T3 or T4.

When the right levels are reached, the thyroid gland then tells the pituitary gland to stop producing TSH. It’s all a balancing act.

“When T3, T4, and TSH are made in appropriate quantities and work properly, they have profound good effects on the body’s tissues,” says Dr. Smita Koppikar, MBBS, DNB, a pediatric endocrinologist in the U.K. and Mumbai, India.

“The workings of the thyroid and pituitary gland are deeply interconnected by a feedback loop, which is how the body beautifully and finely controls the actual amount of T3 and T4 needed,” Dr. Koppikar says.

For this reason, treating a thyroid condition requires an understanding of the complexity of the system. You’ll want to work with an endocrinologist if you are concerned about your thyroid health.

Forms of T4

T4 is found in your body in two forms: 

Free T4

This form of T4 circulates around the body on its own and can freely enter body tissues where it’s needed.

Bound T4 

This form of T4 moves through the body attached (aka bound) to proteins and therefore isn’t able to enter body tissues as easily.

T3 is also found in both the free and bound forms. 

T4 is essentially inactive within the body, explains Yasmin Akhunji, MD, a board-certified endocrinologist at Paloma Health, based in Arizona. T3 is the biologically active hormone that the body can use. However, T4 does play a (very) important role: It is a “storage hormone” that transports T3 to where it’s needed, Dr. Akhunji says.

A healthy thyroid gland releases about 80 percent T4 and only 20 percent T3. Your body organs must convert T4 to T3 before it can be used, which happens in a process called deiodination. 

What Happens When T4 Is Too High or Too Low?

When your body makes too much or too little thyroid hormone, you may experience the symptoms of two main thyroid conditions: hypothyroidism (a condition marked by too little thyroid hormone) or hyperthyroidism (a condition marked by too much thyroid hormone).

High Levels of T4

A high level of T4 (either total T4 or free T4) may indicate hyperthyroidism (an overactive thyroid). Symptoms of this condition include:

  • Weight loss
  • Shaky hands
  • Fast or increased heart rate
  • Diarrhea
  • Nervousness or agitation
  • Sweating
  • Bulging eyes or eyes that appear as if they’re “staring”
  • Insomnia

Low Levels of T4

A low level of T4 (either total T4 or free T4) may indicate hypothyroidism (an underactive thyroid). Symptoms of hypothyroidism include:

  • Weight gain
  • Feeling cold much of the time
  • Dry skin or hair
  • Fatigue or lethargy
  • Low mood
  • Memory problems or brain fog
  • Constipation

Other Endocrine Disorders That Can Alter T4 Levels

If your T4 levels are off, it may also indicate another problem with your endocrine system, such as:

  • Hypopituitarism, which is a condition marked by an underactive pituitary gland. Even though your pituitary gland doesn’t make T4, it does make TSH, which stimulates the production of T4. So low T4 levels might be a sign of hypopituitarism.
  • Trophoblastic disease, which is a rare grouping of tumors related to pregnancy.
  • Thyroiditis, which is inflammation of the thyroid gland
  • A toxic multinodular goiter, which is a growth in the thyroid gland that can result in an overproduction of thyroid hormone
  • Thyrotoxicosis, which occurs when you have way too much T4 in your bloodstream

Outside Variables That Can Affect Your T4 Levels

Finally, a number of external factors can cause changes in your level of free or bound T4. These include:

  • High intake of iodine
  • Long-term illness 
  • Steroid use
  • Taking estrogen, androgen, and other hormone pills 
  • Some other medications, such as cancer drugs and antithyroid drugs
  • Pregnancy (can slightly increase T4)

Testing Your T4 Levels

If you are experiencing symptoms of a thyroid condition, your healthcare provider may want to test your thyroid function. It is common for doctors to test total T4 or the thyroid-stimulating hormone (TSH), although Dr. Akhunji recommends testing TSH, free T3, and free T4 to get a fuller picture of your thyroid health. These tests can help reveal how effective the body is at converting the thyroid hormone into its active form.

Additionally, testing for antibodies to an enzyme called thyroid peroxidase (known as a TPO test) can help detect the presence of a thyroid-related autoimmune condition such as Hashimoto’s thyroiditis or Graves’ disease.

Diagnosis of a thyroid condition isn’t based solely on one specific test, though, so in addition to bloodwork, your healthcare provider will also consider your family history and other potential medical conditions.

In addition to testing your blood levels of thyroid hormone, your healthcare provider may order other tests to see how your thyroid is functioning, such as an ultrasound or biopsy of your thyroid gland. 

What Is A ‘Normal’ T4 Level?

The below ranges for T4 are considered generally normal:

T4: 5.0–11.0 µg/dL

Free T4: 0.9–1.7 ng/dL 

Note that some doctors and labs disagree on exactly what ranges should be considered “normal” or “healthy” when it comes to thyroid tests. So your results may vary slightly depending on the provider.

Treating T4 Imbalances — and Keeping Your Thyroid Healthy Overall

If testing reveals a problem with your T4 or other thyroid levels, your healthcare provider might prescribe medication to get the hormones back in balance. In addition, there are several lifestyle steps you can follow to help keep your thyroid levels balanced, including eating a healthful, thyroid-friendly diet; getting enough sleep; exercising regularly; and reducing stress. Here’s a closer look at some of these areas.


If you are diagnosed with hypothyroidism, you’ll typically get a prescription for a thyroid hormone replacement therapy such as synthetic thyroxine (T4), also known as levythyroxine sodium (brand names include Levo-T and Synthroid). This medication enables the conversion of T4 into T3. Be aware that it may take some time and regular monitoring to get the dosing right.  

If testing indicates you have hyperthyroidism, your healthcare provider may prescribe an “anti-thyroid medication” such as methimazole (brand name Tapazole) or propylthiouracil (also known as PTU). These drugs interfere with the production of thyroid hormones. Your doctor may also prescribe beta-blockers to counter some of the effects of excessive thyroid hormones on your body; for example, these drugs can help slow down a rapid heart rate and reduce hand tremors.

If medication isn’t sufficient to treat your hyperthyroidism or you cannot take anti-thyroid medication, your doctor might recommend other treatments such as radioactive iodine (RAI) therapy or thyroid surgery.


When it comes to maintaining a healthy thyroid, you really are what you eat. Diet can help provide the nutrients and minerals you need to sustain a healthy thyroid, especially in tandem with medication and other lifestyle changes. 

Avoid processed foods and anything with added sugars. Reach for clean foods, like veggies and fruits, dark leafy greens, lean proteins, and seafood.

Eat plenty of foods with selenium (these include eggs, tuna, and legumes) as well as zinc (this includes shellfish, chicken, and whole grains), as both of these minerals help to support the conversion of T4 into T3. Tuna can also support thyroid function due to its high amount of the amino acid tyrosine.

If you are taking levothyroxine (T4), you should avoid eating soy protein, high fiber foods, and walnuts within three or four hours of taking your medication (either before or after), as these may interfere with the absorption of thyroid hormone.


Regular movement is key when it comes to managing your hormones. If your energy levels are low due to hypothyroidism, even walking in place can help you feel more energized. Think biking, jogging, dancing, or even just increasing the distance you walk to and from your car. Studies have found that just 60 minutes of aerobic activity three times per week led to significant quality of life improvements in women with subclinical hypothyroidism (a mild condition that’s considered a precursor to hypothyroidism).

A note of caution: If you have hyperthyroidism, be sure to speak with your healthcare provider before engaging in vigorous exercise. This is because hyperthyroidism speeds up your metabolism on its own. Adding on intense forms of exercise could pose a risk to your heart.

Mental Health

Although thyroid conditions affect you physically, they can also affect your mental health. They may cause anxiety, depression, moodiness, and concentration problems, among other issues. Managing a tricky health condition also adds its own layer of stress. If you are diagnosed with a thyroid condition, it is a good time to turn to a therapist, patient forum, or trusted friend if you are struggling. 

Keeping your nervous system in check can help you feel better physically — while also helping to balance your emotions. In fact, studies have found that patients with Hashimoto's thyroiditis who engaged in stress management interventions saw a significant improvement in anti-thyroglobulin (anti-TG) antibodies.

Try tapping into a pre-bedtime yoga ritual or a stress-relieving hobby, ensuring that you make time for these activities each day. You can also adopt a journaling habit, or fit mindfulness or meditation sessions into your day.