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Simply put, what is thyroxine (T4)?
Thyroxine (T4) is a major hormone produced by the thyroid gland , a butterfly-shaped organ located in the front of our neck, specifically around the windpipe. This small gland controls many important processes in our body.- The rate at which we convert the food we eat into energy ( Metabolism )
- The speed at which food moves through the digestive system
- Our heart rate and body temperature
- Brain development in young children
- How muscles contract
- The rate at which cells in the body die and new cells are formed
Doctor, why are you asking me to do a T4 test?
There are several reasons why your doctor may recommend a T4 blood test.- Diagnose thyroid diseases: This test is important to diagnose conditions such as hypothyroidism or hyperthyroidism and to monitor the condition during treatment.
- Identify problems with the pituitary gland: The pituitary gland in our brain produces a hormone called TSH (Thyroid-Stimulating Hormone). This TSH hormone tells the thyroid gland how much T4 and T3 to make. So, the T4 test also helps to see if there is a problem with the pituitary gland.
- Other causes: Other like TSH or T3If there are abnormalities in thyroid tests, if there is a nodule in the thyroid gland, if the gland is swollen, or if you are having difficulty conceiving a child, you may be asked to do a T4 test to determine exactly how your thyroid is functioning.
| Thyroid condition | Commonly seen symptoms |
|---|---|
| Low T4 levels (Hypothyroidism) Slowing down the body's functioning | Severe depression, loss of memory and attention, swelling around the eyes, hair loss, swelling of the thyroid gland (goiter), extreme fatigue, inability to tolerate cold, weight gain, loss of appetite, dry and cold skin, slow heartbeat, difficulty breathing, constipation, irregular menstrual cycles in women. |
| Increased T4 levels (Hyperthyroidism) Accelerated body function | Frequent irritability and restlessness, insomnia, bulging eyes (Graves' eye disease), weight loss, excessive hunger, heat intolerance, excessive sweating, trembling of the fingers, head, or face, rapid or pounding heartbeat, high blood pressure, diarrhea. |
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Check Availability →Things to know about the T4 blood test
What are Free T4 and Bound T4?
There are two forms of the T4 hormone in our body.- Free T4: This is the T4 that circulates freely in the blood, unbound to anything. These are the T4 that can enter cells and be used at any time.
- Bound T4: These are bound to proteins and travel in the blood. These proteins help the T4 hormone to be transported safely to where it is needed, without being absorbed by other cells along the way. Most of the T4 in our body is bound T4.
Doctors usually look at the Free T4 level because it is the amount of T4 that the body can use, and it gives the best idea of how well the thyroid is functioning.
Things to consider before the test
Before having a blood test, it is important to tell your doctor about any medications, vitamins, or supplements you are taking. Some medications, such as anti-seizure medications, heart medications, and birth control pills, can affect your T4 results. Especially if you are taking a biotin supplement, it can make your T4 levels appear higher than they actually are. This can cause your doctor to miss a thyroid problem. Also tell your doctor if you are pregnant or have other medical conditions, such as kidney disease.Are there any risks to the T4 blood test?
This is a routine test, like any other blood test. A trained person uses a small needle to take a small amount of blood from a vein in your arm and then applies a patch to the area. The risks are very small, but sometimes the following can happen:- A little pain where the needle was inserted.
- A little bleeding
- Bruising (blood pooling under the skin)
- If you don't take care of cleanliness, you could get an infection.
- Dizziness or fainting
Understanding the results of the T4 test
Normal ranges for a Free T4 test may vary slightly depending on age, gender, and the lab that performs the test. However, in general, the normal values are as follows: For pregnant mothers
The most important thing is that only your doctor can interpret the values in this report in a way that suits your situation. Therefore, be sure to take the report and see your doctor.| Age group / status | Free T4 Normal Range (ng/dL) |
|---|---|
| Children up to 5 years old | 0.8 - 2.8 |
| Children 6-15 years old | 0.8 - 2.1 |
| Adults over 18 years old | 0.9 - 1.7 |
| First trimester | 0.8 - 1.53 |
| Second and third trimesters | 0.7 - 1.20 |
Causes of low T4 levels (Hypothyroidism)
- Hashimoto's thyroiditis: This is an autoimmune disease. This means that our own immune system mistakenly attacks the thyroid gland.
- Iodine deficiency in the diet.
- Acute short-term illnesses.
- Malnutrition.
- Some medications.
Causes of increased T4 levels (Hyperthyroidism)
- Graves' disease: This is also an autoimmune disease.
- Excess iodine in the diet.
- Non-cancerous lumps (benign tumors/nodules) in the thyroid gland.
- Swelling of the thyroid gland (Goiter).
- Increasing the dosage of thyroid hormone medication used for treatment.
Additional tests
Although a T4 test can show that there is a problem with your thyroid gland, it cannot tell you exactly what is causing it. So, if your T4 level is abnormal, your doctor will likely order more tests to find the cause.- TSH blood test: This is usually the first test done. TSH and T4 levels are directly related to each other.
- Free T3 blood test: Checks the level of the other main hormone made by the thyroid gland.
- Thyroid antibodies test: Check for a disease like Hashimoto's or Graves'.
- Radioactive iodine uptake imaging: See how the thyroid gland absorbs iodine.
💡 Take-Home Message
- The T4 blood test is an important test that checks how well your thyroid gland is working.
- Low T4 levels are called Hypothyroidism, and high T4 levels are called Hyperthyroidism. Both conditions have related symptoms.
- It is imperative to inform your doctor about all medications and supplements you are taking before the blood test.
- Abnormal T4 levels can be caused by a short-term or long-term problem.
- Always consult your doctor with your test report to find the exact cause and, if necessary, get a proper treatment plan.
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