Thyroxine (T4) levels differ from person to person but usually exist within a consistent range. Abnormal T4 levels can lead to various symptoms and complications, including mental health and digestive issues.

T4 is a type of thyroid hormone that regulates metabolism. T4 plays an important role in digestion, muscle function, and bone health.

In this article, we outline the normal T4 levels in adults and children. We also list some potential causes and symptoms of changes in these levels and outline treatment options.

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T4 levels can affect both bone health and muscle function.

Two separate glands determine T4 levels: the thyroid gland in the neck and the pituitary gland in the brain.

The pituitary gland is responsible for making the thyroid stimulating hormone (TSH). This hormone controls levels of the thyroid hormone T4. The pituitary gland adjusts its production of TSH according to how much T4 it detects in the blood.

For example, if the pituitary gland detects low blood levels of T4, it produces and excretes more TSH into the blood. If it detects high levels of T4, it stops producing TSH. This process ensures that T4 levels stay within a normal range.

There are two types of T4: bound and free. Bound T4 attaches to proteins that prevent it from entering the body’s tissues. Free T4 does not attach to proteins, so it is free to enter body tissues that use it.

A healthcare provider can use two blood tests to determine whether a person’s T4 levels are within a normal range. A total T4 test detects levels of both bound and free T4, while a free T4 test only identifies free T4 levels.

A healthcare provider may also recommend a TSH test to assess thyroid function. Combining a free T4 test with a TSH test gives the most accurate insight into how the thyroid is functioning.

Normal levels in adults

In adults, normal levels of total T4 range from 5–12 micrograms per deciliter (mcg/dl) of blood. Normal levels of free T4 range from 0.8–1.8 nanograms per deciliter (ng/dl) of blood.

Normal T4 levels also vary across trimesters of pregnancy. A 2011 study in the Journal of Thyroid Research outlines the normal ranges for pregnant women as 0.95–1.53 ng/dl in the first trimester and 0.87–1.45 ng/dl in the second trimester.

Normal levels in children

For children, normal T4 ranges vary according to age.

The table below shows the normal range for free T4 levels.

AgeLower limit in picomoles (pmol)Upper limit (pmol)
less than 6 days1132
6–13 days11.528.3
14 days or older1222

Although the normal T4 range is wide, levels usually remain stable for each person. A T4 value at the low or high end of normal for one person may be typical for another.

Sometimes a person’s T4 levels may change and fall outside the normal range. Depending on the direction of change, this can result in hypothyroidism or hyperthyroidism.

Hypothyroidism is the medical term for an underactive thyroid gland that does not produce enough hormone. Conversely, hyperthyroidism is when an overactive thyroid gland produces too much hormone.

Several factors can cause a change in T4 levels.

Causes of hypothyroidism

Below are potential causes of hypothyroidism:

Hashimoto’s thyroiditis

In this autoimmune condition, the immune system mistakenly attacks the thyroid gland. Eventually, the damaged thyroid is unable to produce enough thyroid hormones. Hashimoto’s thyroiditis is the most common cause of hypothyroidism in the United States.


Thyroiditis is an inflammation of the thyroid gland. It causes thyroid hormones to leak into the blood, raising their overall levels and leading to hyperthyroidism. After 1–2 months, this may develop into hypothyroidism. Thyroiditis can be due to a viral or bacterial infection, an autoimmune condition, or pregnancy.

Previous thyroid treatment

Certain treatments for hyperthyroidism and thyroid cancer can cause side effects or complications that lead to hypothyroidism. Examples of these treatments include surgery or radioactive iodine therapy.

Congenital hypothyroidism

This condition occurs when the thyroid gland does not function properly from birth. It can lead to physical and mental growth issues, but early treatment can prevent these complications.

Causes of hyperthyroidism

Below are potential causes of hyperthyroidism:

Graves’ disease

Graves’ disease is an autoimmune condition where the immune system mistakenly attacks the thyroid gland, causing it to overproduce thyroid hormone.

Thyroid nodules

Thyroid nodules are lumps that can develop on the thyroid gland. Some contain thyroid tissue, which contributes to the overproduction of thyroid hormone. Most thyroid nodules are benign, but some may be cancerous.

Excessive iodine intake

A person who takes additional iodine in supplements or medicines can cause their thyroid gland to produce too much of the hormones.

The symptoms of abnormal T4 levels vary according to whether the thyroid produces too little T4 or too much.

Symptoms of hypothyroidism

In hypothyroidism, the thyroid gland produces too little T4, and metabolism slows down. This can lead to the following symptoms:

Symptoms of hyperthyroidism

In hyperthyroidism, the thyroid gland produces too much T4, and metabolism speeds up. This can lead to the following symptoms:

Thyroid conditions share the same symptoms as other diseases. As such, doctors usually cannot diagnose abnormal T4 levels based on symptoms alone. Instead, they may use thyroid function tests (TFTs) to measure levels of thyroid hormones in the blood.

TFTs usually involve testing levels of the following thyroid hormones:

  • thyroid-stimulating hormone (TSH) or thyrotropin
  • free T4 or thyroxine
  • free T3 or tri-iodothyronine, another type of thyroid hormone

Normal thyroid hormone levels fall within a wide range. As such, it can sometimes be difficult to determine whether a person’s thyroid hormone levels are at the lower or higher end of normal. A combination of the above tests can provide a doctor with a clearer picture of how a person’s thyroid gland is functioning.

Home thyroid tests also measure T4 along with other thyroid hormones.

The treatment for abnormal T4 levels varies depending on whether the levels are too low or too high.

Treatment for hypothyroidism

About 4.6% of U.S. citizens aged 12 years and older have hypothyroidism, and most of these cases are mild. Most people who develop the disease are over 60 years of age.

To treat this condition, a person takes the synthetic hormone levothyroxine daily to increase levels of the natural thyroid hormone thyroxine.

When a person first starts taking levothyroxine, they require blood tests every 6–8 weeks. These allow a doctor to monitor their T4 levels so they can adjust the dosage if necessary. Once a doctor establishes the correct dosage, a person may require follow-up blood tests every 6–12 months.

Hypothyroidism is usually manageable with the correct medication. A person must continue to take their dosage unless a doctor advises them otherwise.

Treatment for hyperthyroidism

Several different treatments are available for hyperthyroidism. A doctor will prescribe the best option based on a person’s individual needs.

Treatment options include:

  • Medication: Medications called thionamides stop the thyroid from producing too many hormones.
  • Radioiodine therapy: This type of radiotherapy destroys the cells in the thyroid gland, reducing the level of hormones that the thyroid produces.
  • Surgery: As a last resort, a doctor may recommend surgery to remove a part or all of the thyroid gland. If they remove part of the gland, a person’s T4 levels may return to normal. If they remove the entire gland, a person will need to take thyroid hormone medication for the rest of their life.

A person with abnormal T4 levels is likely to require some form of treatment to bring their levels back to within a normal range. Some people need to keep taking medication for several years or possibly for life.

A person also requires routine blood tests to monitor their T4 levels over time.

With the right treatment, most people can normalize their T4 levels and go on to live a relatively normal life.

T4 is a hormone that the thyroid gland produces. It regulates metabolism and plays a crucial role in digestion, muscle function, and bone health.

T4 levels exist within a normal range. Hypothyroidism is where the thyroid produces too little thyroid hormone, which results in lower-than-normal T4 levels.

In people with hyperthyroidism, the thyroid gland produces too much thyroid hormone, which leads to higher-than-normal T4 levels.

Either condition can result in a wide range of symptoms and complications.

Thyroid function tests are blood tests that measure thyroid hormone levels and can identify abnormal thyroid function.