Blood test

High T4 level in the blood

Blood test
High T4 level in the blood
Last update: 21/10/2020

What is high level of total T4 (Thyroxine) in the blood called?

  • Hyperthyroxinemia

What is the normal T4 level in the blood?

If you need to know which are the T4 reference ranges or you require more information about the role of T4 in the blood, you can visit normal T4 level in the blood

What does a high total T4 level in the blood mean?

A T4 (Thyroxine) level in the blood above the normal range may be a sign of hyperthyroidism, a disease where the thyroid gland produces too much thyroid hormones.

T4 values should be studied along with the values of T3 hormone (Triiodothyronine) and TSH hormone (thyroid-stimulating hormone) for a better diagnosis.

There are two types of hyperthyroidism and it is important to study TSH values to discern them:

  • Primary hyperthyroidism: due to a hyperfunction of the thyroid gland. TSH values will be below the normal range.
  • Secondary hyperthyroidism: due to a failure of the pituitary gland. TSH values will be above the normal range.

T4 blood values are usually given in µg/ml but sometimes you can see these values in nmol/l following the International System of Units (SI). In case your values are in nmol/l you can convert them using this tool:


Hyperthyroxinemia or high T4 blood levels mean:

  • Marked hyperthyroxinemia (> 12 µg/dl in adults) :

    T4 (thyroxine) values are very high and it may be the consequence of suffering from hyperthyroidism, a hyperfunction of thyroid gland.

    If the TSH (thyroid-stimulating hormone) value is above the normal range is a sign of primary hyperthyroidism where there is a problem of the thyroid gland.

    It can be advisable to study free T4 in the blood to get more information about the disorder.

    When the T4 level in the blood is higher than normal, you may suffer from irritability and weight loss. You should visit your doctor or an endocrinologist to prescribe you a proper treatment.

Which factors can raise the T4 level in the blood?

There are some health circumstances or drugs than can raise your T4 level in the blood:

Which diseases can raise your T4 level in the blood?

The following diseases may raise the T4 level in the blood:

  • Struma ovarii
  • Molar pregnancy

What can I do to lower the T4 level in the blood?

There are some antithyroid drugs, such as carbimazole, that may reduce your T4 level in the blood. They should be used only under medical prescription.

Another possible treatment, if your doctor considers it appropriate, is thyroidectomy (surgical removal of all or part of the thyroid gland) following by oral synthetic T4 (Levothyroxine).

Where can I find more information about T4 level in the blood?

You can visit our pages about:

Which values are considered a high T4 level in the blood?

The following values are considered to be above the normal range:

IMPORTANT: These levels are expressed in µg/dl. They are an example of a healthy man of about 45 years old with no known disease and not taking any medication. The ranges can be different depending on the laboratory or on your personal circumstances.

Marked hyperthyroxinemia
12.1 µg/dl12.2 µg/dl12.3 µg/dl12.4 µg/dl12.5 µg/dl12.6 µg/dl12.7 µg/dl12.8 µg/dl
12.9 µg/dl13 µg/dl13.1 µg/dl13.2 µg/dl13.3 µg/dl13.4 µg/dl13.5 µg/dl13.6 µg/dl
13.7 µg/dl13.8 µg/dl13.9 µg/dl14 µg/dl14.1 µg/dl14.2 µg/dl14.3 µg/dl14.4 µg/dl
14.5 µg/dl14.6 µg/dl14.7 µg/dl14.8 µg/dl14.9 µg/dl15 µg/dl15.1 µg/dl15.2 µg/dl
15.3 µg/dl15.4 µg/dl15.5 µg/dl15.6 µg/dl15.7 µg/dl15.8 µg/dl15.9 µg/dl16 µg/dl
16.1 µg/dl16.2 µg/dl16.3 µg/dl16.4 µg/dl16.5 µg/dl16.6 µg/dl16.7 µg/dl16.8 µg/dl
16.9 µg/dl17 µg/dl17.1 µg/dl17.2 µg/dl17.3 µg/dl17.4 µg/dl17.5 µg/dl17.6 µg/dl
17.7 µg/dl17.8 µg/dl17.9 µg/dl18 µg/dl18.1 µg/dl18.2 µg/dl18.3 µg/dl18.4 µg/dl
18.5 µg/dl18.6 µg/dl18.7 µg/dl18.8 µg/dl18.9 µg/dl19 µg/dl19.1 µg/dl19.2 µg/dl
19.3 µg/dl19.4 µg/dl19.5 µg/dl19.6 µg/dl19.7 µg/dl19.8 µg/dl19.9 µg/dl20 µg/dl
Medically reviewed by our Medical staff on 21/10/2020


  • Approved Guideline – Procedures for the Handling and Processing of Blood Specimens, H18-A3. 2004. Clinical and Laboratory Standards Institute.
  • HHS Publication, 6th ed., June 2020. Biosafety in Microbiological and Biomedical Laboratories. Available on:
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