fT4 (Free Thyroxine) Test: What does it mean?

What is fT4?

T4 is a hormone made by the thyroid gland. It is converted into the active form, T3, in different body parts like the liver, kidneys, muscles, and brain. The triiodothyronine (T3) and thyroxine (T4) hormones are important for many bodily functions including metabolism, heart and digestive function, muscle control, brain development, and bone maintenance.

What is the purpose of performing an fT4 test?

• To check if the thyroid is working properly.
• To check if the pituitary gland is working properly.
• To identify symptoms of thyroid problems like weight changes, irregular heartbeats, nervousness, menstrual irregularities, dry skin, muscle pain, and infertility.
• To monitor T4 levels if taking thyroid hormone replacement therapy.

What does it mean if my fT4 results are low?

Hypothyroidism, characterized by low T4 levels, is commonly caused by an underactive thyroid gland. This condition can be attributed to various factors, including autoimmune diseases, insufficient dietary intake of iodine, and certain medications. Individuals with hypothyroidism may experience symptoms such as weight gain, fatigue, sensitivity to cold temperatures, and irregular or heavy menstrual periods.

Low result causes

• Hashimoto’s thyroiditis (This condition refers to the underactive thyroid or hypothyroidism caused by an autoimmune process).
• Thyroiditis (inflammation of the thyroid gland).
• Possible thyroid cancer.
• Pituitary disease ( This condition involves the disruption of pituitary gland function, which plays a role in regulating the thyroid gland).

What does it mean if my fT4 results are high?

Elevated T4 or free T4 levels may indicate thyrotoxicosis—which can occur due to various conditions such as hyperthyroidism (overactive thyroid), thyroid inflammation (thyroiditis), or excessive thyroid medication intake.

Thyrotoxicosis speeds up your metabolism—which can be dangerous to your health. Individuals with thyrotoxicosis may exhibit symptoms like weight loss, increased appetite despite eating more, rapid or irregular heartbeat, nervousness, irritability, difficulty sleeping, fatigue, shaky hands, muscle weakness, excessive sweating, heightened sensitivity to heat, frequent bowel movements, Graves’ disease, thyroiditis, thyroid cancer, or the presence of thyroid nodules (multinodular goiter).

Note:

Your hormonal level results are not a substitute for medical advice. Please consult with your healthcare provider.

How can I test fT4?

fT4 is a blood test. You have two options to obtain results: 

• Clinical blood test from a vein,
• At-home finger-prick test, that is gaining popularity due to their convenience and privacy.

To get your fT4 results, you can use Mira Panorama Fertility Lab Test (US only).

It’s a quick and easy at-home finger-prick test that measures 5 hormones: not only fT4, but also AMH, TSH, Prolactin, and Free Testosterone. 

You will get 5 individual and personalized hormone reports with actionable steps to understand what your numbers mean for your ovarian reserve, thyroid function, menstrual cycle, hormonal balance, and fertility health. 

Resources

Payne, A. H., & Hardy, M. P. (Eds.). (2015). Thyroid-Stimulating Hormone. In Molecular, Genetic, and Cellular Endocrinology of the Testis (Vol. 5). Springer.

U.S. National Library of Medicine. (n.d.). T4 (Thyroxine) Test. MedlinePlus. Retrieved from https://medlineplus.gov/lab-tests/thyroxine-t4-test/

University of Rochester Medical Center. (n.d.). Free T4 (Thyroxine). Health Encyclopedia. Retrieved from https://www.urmc.rochester.edu/encyclopedia/content.aspx?contenttypeid=167&contentid=free_t4_thyroxine

ScienceDirect. (n.d.). Free Thyroxine Index. In Biochemistry, Genetics, and Molecular Biology. Retrieved from https://www.sciencedirect.com/topics/biochemistry-genetics-and-molecular-biology/free-thyroxine-index

Hollenberg, A. N. (2022). Thyroxine and the Brain: A Brief Overview and Consequences of Thyroxine Overdose in Adults. Drug Development Research, 1-10. https://doi.org/10.1080/10408363.2022.2121960