Blood test

Low T4 level in the blood

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

What is a low T4 level in the blood called?

  • Hypothyroxinemia

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 low total T4 (Thyroxine) level in the blood mean?

Values of T4 (Thyroxine) below the normal range may be a sign of suffering from hypothyroidism, a disorder where the thyroid hormone fails to produce enough 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 hypothyroidism that can be discerned by studying the TSH:

  • Primary hypothyroidism: due to a hypofunction of the thyroid gland. TSH values will be above the normal range. It is the most common situation.
  • Secondary hypothyroidism: due to a failure of the pituitary gland. TSH values will be below 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:


Hypothyroxinemia or low T4 blood levels mean:

  • Marked hypothyroxinemia (< 4 µg/dl in adults):

    Your total T4 blood level is very low and it may be a sign of hypothyroidism.

    If the TSH (thyroid-stimulating hormone) value is above the normal range is a sign of primary hypothyroidism. If the TSH value is below the normal range is a sign of secondary hypothyroidism.

    A free T4 blood test may be asked because it can give more detailed information about the disorder than the total T4.

    If your T4 level in the blood is low, you may suffer from fatigue or weight gain. You should visit your doctor or an endocrinologist to prescribe you a proper treatment.

    In case of a low T4 level in a newborn, it may be a sign of congenital hypothyroidism.

Which factors can reduce the total T4 level in the blood?

Some particular health situation or drugs may reduce your T4 level in the blood:

Which diseases can reduce your total T4 level in the blood?

The following diseases can explain a total T3 level in the blood lower than normal:

What can I do to increase the total T4 level in the blood?

If your T4 (Thyroxine) level in the blood is lower than normal the following tips can help you:

  • Increase your iodine intake in your diet because it can be a consequence of an iodine deficiency. That is not usually a problem in developed countries. You can eat more of iodized salt, cod, tuna, chicken or broccoli.
  • You can take, under medical prescription, levothyroxine (a manufactured form of the thyroid hormone thyroxine T4) to increase your T4 level to optimal levels.

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

You can visit our pages about:

Which values are considered a low 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 hypothyroxinemia
3.9 µg/dl3.8 µg/dl3.7 µg/dl3.6 µg/dl3.5 µg/dl3.4 µg/dl3.3 µg/dl3.2 µg/dl
3.1 µg/dl3 µg/dl2.9 µg/dl2.8 µg/dl2.7 µg/dl2.6 µg/dl2.5 µg/dl2.4 µg/dl
2.3 µg/dl2.2 µg/dl2.1 µg/dl2 µg/dl1.9 µg/dl1.8 µg/dl1.7 µg/dl1.6 µg/dl
1.5 µg/dl1.4 µg/dl1.3 µg/dl1.2 µg/dl1.1 µg/dl1 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:
  • Concise Book of Medical Laboratory Technology: Methods and Interpretations. 2nd Edition. 2015. Ramnik Sood. ISBN: 978-93-5152-333-8. Pag. 783.
  • Tietz. Fundamentals of Clinical Chemistry. Carl A. Burtis, Edward R. Ashwood, David E. Bruns, Barbara G. Sawyer. WB Saunders Company, 2008. Pag 766. ISBN: 978-0-7216-3865-2.
  • Laboratory tests and diagnostic procedures with nursing diagnoses (8th ed), Jane Vincent Corbett, Angela Denise Banks, ISBN: 978-0-13-237332-6, Pag. 400.

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