Did you know that free T3 is the most powerful thyroid hormone?
Your thyroid is at the heart of your energy, metabolism, and overall vitality, with free T3 standing as its most powerful hormone. Understanding the role of free T3—a biologically active hormone that directly drives energy production at the cellular level—can be transformative for your health. By uncovering what influences free T3 levels, from nutrient intake to lifestyle choices, you gain the knowledge to take actionable steps toward better thyroid function and well-being. Empower yourself to harness the full potential of your thyroid and reclaim control over your health today.
What is free T3?
Free T3, also known as triiodothyronine, is a crucial thyroid hormone that directly impacts your energy levels, metabolism, and overall well-being. Unlike total T3, free T3 refers to the active form of the hormone that is not bound to proteins, making it readily available for your body to use. Monitoring free T3 levels is essential for understanding thyroid health and addressing any imbalances effectively.
The difference between free T3 and T3.
Your free T3 lab test is a measurement of how much free T3, also called FT3, is in your bloodstream. This is different from your T3 blood test. To make sense of this, we need to talk about what makes T3 free or not free. If the hormone has a protein attached to it, then it is not free. We call T3 with a protein, a bound hormone and T3 without a protein a free hormone.
Your T3 blood test is measuring your free T3 + your bound T3. Your free T3 test blood test only measures the T3 that is free.
T3 = bound and free T3
FT3 = free T3 only
Why is free T3 the most powerful thyroid hormone?
When your T3 hormone is in a free state, it is able to engage with your cells. If it is attached to a protein (I will talk more about these proteins later in this article) it is not permitted access to your cells. If thyroid hormone can’t get inside your cells, it can’t do its job. Only about 30% of T3 is in the free state; the other 70% is rendered biologically inactive…until it is freed from its protein. Your T3 free lab test is only a small portion of your total T3… about 30%. The reason free T3 is the most powerful thyroid hormone is because it is the most biologically active form of thyroid hormone. It is more active than T3, T4 or free T4.
The difference between T3 and T4 thyroid hormones.
Your T3 lab test is different from your T4 lab test. What is the difference? I cover T4 in great detail in its own article, please refer to it if you want to know all about T4…ok back to T3. Your thyroid gland makes mostly T4 hormone, in fact, 90% of thyroid hormone is in the form of T4. We call it T4 because there are 4 atoms of iodine attached to it. There are special enzymes called deiodinase enzymes (see below) that remove one of the iodine atoms and T4 becomes T3. Technically, T4 is what we call a prohormone, which means it is a hormone that gets converted into a different, typically smaller (one less iodine atom) biologically active hormone.
The purple dots are the iodine atoms. There is a form of T3 called reverse T3. Reverse T3 is not biologically active and we will cover it in another article.
What does free T3 do?
Free T3 is essential for maintaining energy production and metabolic balance within your body. Low levels can disrupt cellular function, leading to symptoms like fatigue, weight gain, and other health issues. It works with TSH to regulate thyroid hormone production, highlighting its critical role in overall well-being.
Your metabolism and free T3
Free T3 hormone enters your cells and stimulates the production of energy. If your levels of thyroid hormone are low, your cells make less energy and your metabolism will slow down. If your cells can’t make enough energy to function properly they cannot maintain basic cell functions and the health of the cell declines. This slow down is what leads to the signs and symptoms of low thyroid function (hypothyroidism). Low levels of thyroid hormone can cause:
- Hair loss
- Weight gain
- Fatigue
- Brain-fog
- Irregular/heavy menstrual cycles
- Depression
- infertility
- Digestion problems
- Dry skin
- Unhealthy fingernails
- Abnormal cholesterol levels
- And many other common health problems!
T3 influences thyroid hormone production
How does free T3 control thyroid hormone production? Well, it doesn’t do this all by itself, it works with another hormone called TSH or thyroid-stimulating hormone. Your TSH thyroid blood test measures the hormone called TSH and this is the hormone that tells your thyroid gland to make hormones. TSH is produced in your pituitary gland. So what does T3 have to do with TSH? If the levels of free T3 are low in the pituitary gland, it will make more TSH which informs your thyroid gland to make more thyroid hormones (T4 and T3).
There is another hormone called TRH that is also involved in TSH regulation but I don’t want to get too far off track here and I have covered TRH in its own article. Ok, so your blood lab TSH is directly influenced by the level of free T3 in the pituitary gland and TSH tells your thyroid gland to make more thyroid hormone. Let’s move on to what can affect your free T3 levels.
What can change your free T3 levels?
Thyroid hormones circulate in the blood, bound to proteins like TBG. Factors such as pregnancy and hormone therapies can elevate TBG, which can affect total thyroid hormone levels. Additionally, enzymes that convert T4 into active T3 play a crucial role in thyroid function.
Proteins in your blood
Remember at the beginning of this article I spoke of proteins being bound to your thyroid hormone? Well here are those proteins. If the levels of proteins attached to thyroid hormone are too high to too low, it will affect your thyroid on blood tests. There are 3 main proteins to consider, but the one I want to focus on is called TBG/ thyroxine-binding globulin (globulin means protein). Elevated levels of TBG can cause your T3 and T4 on lab tests to be elevated. More of these proteins in the blood can also cause more of your T3 to be bound and not in the free state. The most common causes for elevated TBG are pregnancy, estrogen therapies and oral birth control.
Enzymes and free T3 levels
Your body has special enzymes that convert T4 into T3 and we call them deiodinase enzymes. These are enzymes that remove one iodine atom so that T4 gets turned into T3. These enzymes contain selenium and this is one reason why selenium is so important in thyroid health.
Anything that affects how well these enzymes work will directly affect your levels of T3 and free T3. Most of your T3 is produced by these enzymes in your liver, kidneys and other tissues. If your levels of T4 are normal and your levels of free T3 or free T3 are low, then your enzymes may not be working well. There are several possible causes for poor enzyme function including chronic health issues, stress, other hormone imbalances, food restriction or starvation.
Do you have to fast for thyroid blood work?
Many people wonder if they need to fast before a free T3 thyroid lab test. It is typically not necessary to fast before a thyroid blood test. It is however recommended that you do not take any supplements containing biotin 72 hours before a thyroid test.
Summary & Conclusion
- Thyroid hormones influence energy production and metabolism.
- Your T3 comes from T4.
- Special deiodinase enzymes convert T4 into T3 by removing 1 iodine atom.
- T3 can be free or bound.
- Free T3 is your most biologically active thyroid hormone.
- Free T3 concentrations within the pituitary gland influence the level of TSH which is the hormone that stimulates the thyroid gland to make thyroid hormones.
- Other health issues or medications can alter your levels of carrier proteins that bind to your thyroid hormones.
It is important to realize that other health issues, nutritional deficiencies, inflammation, chronic stress and various lifestyle variables can throw off the delicate balance of the thyroid gland. This can result in changes in your TSH, T4, free T4, T3 and your free T3. Your thyroid gland does not function in isolation, it is influenced by other organs and systems of the body.
Get Your At Home Free T3 Blood Test
Checking your free T3 levels is crucial for understanding thyroid health and energy metabolism. As the most active thyroid hormone, free T3 plays a key role in regulating your body’s energy production. By ordering an at-home free T3 thyroid test, you can easily track your thyroid function and make informed decisions about your health.
I hope you found this article helpful.
Additional Thyroid Blood Tests
I will be writing blogs and making videos for all of the major thyroid tests. This will include TRH, TSH, T4, T3, rT3, T3U, TPOAb, TSI, TgAb and TRAb.
This article by Dr. Clint Sexton and Sexton Functional Medicine is for educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. Always consult a qualified healthcare provider regarding your individual health concerns. Reading this article does not establish a doctor–patient relationship.
