Thyroxine (T4) is one of the two major thyroid hormones that controls how your body uses energy. T4 is produced by the thyroid gland from tyrosine and iodine, and production is regulated by the pituitary gland- a small gland located at the base of the brain. On its own, T4 is inactive in the body. T4 is converted to the active hormone triiodothyronine (T3) in the liver, gut, heart, and muscles, which allows the hormone to perform functions in the body. T4 is one of the thyroid hormones that play an important role in various bodily processes, such as metabolism, blood flow, and body temperature.
What is the T4 hormone?
T4 is the main hormone secreted from the thyroid gland into the bloodstream. Most of the T4 in the blood is bound to proteins and is therefore unusable by the body’s tissues. A limited amount of T4 is unbound to proteins and can be converted into T3. This is known as free T4.
The difference between T4 and FT4
Most T4 is bound to proteins in the blood, and is not usable by the body tissues. Free T4 (FT4) is the T4 in the blood that is not bound to proteins and is therefore usable by the body.
Where is T4 made?
T4 is made by the thyroid gland. The thyroid makes and releases two hormones; T3 and T4. The level of T3 and T4 in the blood is controlled by thyroid stimulating hormone (TSH), a hormone that is produced in the pituitary gland. If T4 or T3 levels fall too low, TSH stimulates the thyroid to produce and secrete more of these hormones.
Why is T4 important?
T4 is an important hormone because it affects all kinds of body processes after its conversion to T3. T4 has an effect on the metabolism, including blood flow, body temperature, and growth.
T4 influences the following body processes:
- Blood flow
- Body temperature
- Growth hormone
Source: Cleveland Clinic, 2022
What are normal T4 values?
Your T4 value is related to how well your thyroid is functioning. FT4 and T4 levels can be checked using a blood test. This can be performed at your GP or using a home test. The normal range for T4 is 5 to 12 micrograms per deciliter (mcg/dL) (UCLA Health). The normal range for FT4 is 11.60 – 12.90 picomoles per liter (pmol/L). If your T4 or FT4 levels are outside this range, see your doctor.
Symptoms of T4 deficiency
Low T4 can be caused by diseases affecting the thyroid, certain medications, or an underactive thyroid (hypothyroidism). Symptoms of hypothyroidism include:
- Weight gain
- Sensitivity to cold
- Brittle hair and nails
- Muscle cramps and weakness
- Irregular periods
Symptoms of high T4
High T4 can be caused by thyroid inflammation, excess iodine, thyroid nodules, or an overactive thyroid (hyperthyroidism). Symptoms of hyperthyroidism include:
- Difficulty sleeping
- Nervousness, anxiety, or irritability
- Mood swings
- Muscle weakness
- Sensitivity to heat
- Persistent thirst
The consequences of abnormal T4 values
Both too high and too low T4 can indicate dysfunction with the thyroid and cause unpleasant symptoms. Untreated hyper- and hypo-thyroidism can lead to serious health problems, including heart problems, blood clots, and high cholesterol. Luckily, irregular thyroid hormones can be treated. If your T3 or T4 level is abnormal, consult your doctor regarding next steps.
What should I do if my T4 level is abnormal?
If you have performed a T4 test at the doctor, they will interpret the test results and provide follow-up advice if needed. If you have performed a home thyroid test, take the laboratory report to your GP for further evaluation. All Homed-IQ test results come with a downloadable laboratory report.
NHS website. (2021, November 18). Symptoms. nhs.uk. https://www.nhs.uk/conditions/overactive-thyroid-hyperthyroidism/symptoms/
NHS website. (2021, November 18). Symptoms. nhs.uk. https://www.nhs.uk/conditions/underactive-thyroid-hypothyroidism/symptoms/
Normal Thyroid Hormone Levels – Endocrine Surgery. (n.d.). UCLA Health. https://www.uclahealth.org/medical-services/surgery/endocrine-surgery/conditions-treated/thyroid/normal-thyroid-hormone-levels
T3 (Triiodothyronine) Test: What It Is, Function & Levels. (n.d.). Cleveland Clinic. https://my.clevelandclinic.org/health/diagnostics/22425-triiodothyronine-t3
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