By Jillian Foglesong Stabile MD, FAAFP, DABOM
The thyroid gland lives at the base of your neck and plays a vital role in your metabolism. The thyroid gland produces three hormones and there are several more involved in its regulation.[1] Two of the hormones produced by the thyroid are T3 and T4. Understanding thyroid hormones and how they influence your body can help you take control of your health. So let's discuss T3 and T4 hormones to better understand what they are and how they compare.
Thyroid Hormone Production
There are three types of hormones produced by the thyroid gland: T3 or triiodothyronine, T4 or thyroxine, and calcitonin. Calcitonin is made by C-cells located within the thyroid gland, is involved in calcium and bone metabolism, and isn't necessarily considered a proper thyroid hormone.[1] The other two hormones, the T3 and T4 hormones, are proper thyroid hormones involved in multiple metabolic functions in the body.
The thyroid gland makes thyroid hormones in response to input from the pituitary gland.[2] The hypothalamus also plays a role in the feedback loop, which regulates thyroid hormone production.[1] The regulation of thyroid hormones is a complex process.
The thyroid gland requires iodine to produce thyroid hormones. Iodine must come from your diet. It cannot be manufactured in your body.[3] Iodine deficiency in the United States is rare because there are many foods that are supplemented with iodine, such as salt.
T4 is produced exclusively in the thyroid gland, while T3 is produced in the thyroid gland but also in other tissues.[3] The thyroid gland contains multiple follicles which serve as the site of thyroid hormone production. The pituitary gland produces a hormone called thyroid stimulating hormone (TSH), which prompts the follicles in the thyroid gland to produce more thyroid hormone.[4]
T4 is produced at higher rates than T3, and T4 is less biologically active than T3. T4 is converted into the more active T3 by multiple tissues in the body by removing iodine molecules.[3] Both T4 and T3 can be bound to proteins and free, though the protein-bound form of both hormones is predominant in the body. It is the free form of thyroid hormones that are responsible for the majority of the functions of thyroid hormone.[3]
Another type of thyroid hormone you may read about is reverse T3 (rT3). Rt3 is produced by the thyroid gland but is also created when T4 is broken down and doesn't have much of a role in the body so it isn't generally monitored.[5]
| Triiodothyronine (T3) | Thyroxine (T4) | |
|---|---|---|
| Site of Production | Thyroid (10%) and other tissues (by conversion from T4) | Thyroid (90%) |
| Amount produced | Lower | Higher |
| Activity | More active | Less active |
| Binding | Free and protein-bound | Free and protein-bound |
| Binding to thyroid hormone receptors | Stronger | Weaker |
| Number of iodine atoms | 3 | 4 |
| Functions[6] | -Bone and connective tissue development before birth -controls resting respiratory rate -affects growth hormone production -controls ovarian cycle -affects protein synthesis |
-impacts heart function -Impacts muscle function -controls metabolism and weight regulation -Maintains bone health -helps the developing brain |
Thyroid Hormone Function
The thyroid gland releases T4 into the bloodstream and other tissues in the body remove iodine molecules to convert T4 to the more active T3. This conversion occurs in cells in [7]:
- Thyroid
- Muscles
- Pituitary gland
- Brown fat tissue
- The brain and central nervous system
- Kidneys
- Liver
While T3 is considered the active form of thyroid hormone, both T3 and T4 hormones regulate several systems in the body.[7]
In the heart, thyroid hormones affect the heart rate as well as the amount of blood the heart pumps, and how hard the muscles of the heart contract. In the lungs, thyroid hormones may stimulate the respiratory centers to increase oxygenation.[1]
In addition to how thyroid hormone increases the contraction of the heart muscles, it also affects how other muscles in your body contract.[1]
Thyroid hormones also play a significant role in metabolism by helping to control basal metabolic rate. Thyroid hormones affect your rate of digestion as well as how effectively your body uses calories.[7] Thyroid hormones don't directly affect your blood sugar levels, but they can affect how your body absorbs and creates glucose levels. Thyroid hormones can also affect the building and breakdown of lipids.[1]
Thyroid hormones play a very important role in the brain. They are vital to brain development and also brain function. Thyroid disease is associated with depression and other mental health conditions and also affects energy levels. People with low thyroid disease can have decreased reflexes and coordination.[8] Abnormal thyroid hormone levels are also associated with hallucinations, memory problems, and even psychotic behavior in some cases.[9] In children, low levels of thyroid hormone are associated with developmental delay.
Thyroid Hormone Imbalances
Since thyroid hormones play several important roles in your body, imbalances in thyroid hormones can have a number of different symptoms.
Hyperthyroidism
Hyperthyroidism occurs when your thyroid gland is overreactive and produces too much thyroid hormone. High levels of thyroid hormone can cause a number of symptoms including [10]:
- Rapid heartbeat/palpitations
- Anxiety or feelings of nervousness
- Hyperactivity
- Mood swings
- Heat sensitivity
- Insomnia
- Itching
- Diarrhea
- Weakness
- Weight gain or weight loss
- Weakness of the bones
Hyperthyroidism is associated with elevated levels of T3 or T4 with suppressed levels of TSH.
Hypothyroidism
Hypothyroidism is an underactive thyroid gland. The thyroid doesn't produce enough thyroid hormone. Hypothyroidism is associated with [11]:
- Weight gain
- Muscle and joint pain
- Cold sensitivity
- Dry skin or hair
- Hair loss
- Heavy or irregular periods
- Infertility
- Miscarriage
- Depression
- Memory loss
- Slow heart rate
- Brain fog
Hypothyroidism is the most common form of thyroid disease seen in patients. In some cases, it can cause severe swelling (myxedema) and even coma.[12]
Graves' Disease
Graves' disease is a specific type of autoimmune thyroid disease. It is one of the most common causes of hyperthyroidism.[13] In Grave's disease, the body attacks the thyroid gland and causes the thyroid to produce too much hormone. It can cause swelling of the eyes and shins, affect vision, and cause swelling of the neck (goiter).
Hashimoto's Disease
Hashimotos' disease is another type of autoimmune thyroid disease, also called chronic lymphocytic thyroiditis. It is the most common cause of hypothyroidism and is genetic in nature. Hashimotos' is more common in older people and women.[14] Hashimoto's disease may initially cause hyperthyroidism as the antibodies attack the gland, causing inflammation, but as the disease progresses, the thyroid gland is destroyed, and hypothyroidism occurs. The symptoms of this disease are generally the symptoms of hypothyroidism, though even when the hypothyroidism is treated, patients may still have symptoms.
Tests for Thyroid Function
There are many tests that can be done for thyroid function.
T3 and T4 hormones can be tested directly in both protein-bound and free forms.[15,16] Elevated T3 or T4 levels can indicate hyperthyroidism while low T3 or T4 levels may indicate hypothyroidism. T3 is slower to respond to changes in thyroid conditions, so T4 is tested more often than T3.[16] Reverse T3 levels are not commonly tested and may be used more often in the functional medicine space.[5]
TSH is usually the most common test used to monitor thyroid function.[17] As mentioned previously, TSH is released by the pituitary gland in the brain and travels to the thyroid gland to stimulate the release of thyroid hormones. Low TSH levels may indicate hyperthyroidism, while high TSH levels may indicate hypothyroidism.[1]
Graves' disease and Hashimoto's disease are both autoimmune diseases. Autoimmune diseases result in elevated thyroid autoantibody levels. These antibodies may be tested in the blood. Thyroglobulin antibody and thyroid peroxidase antibody may be elevated in both Hashimoto's and Graves' among other conditions. Thyroid-stimulating antibody is most commonly elevated in Graves' disease.[18]
Another test that is sometimes conducted is thyroid hormone-binding globulin (TBG). TBG is a protein that is made in the liver and reversibly binds T3 and T4. The elevation in TBG levels may increase total T4 and T3, but it does not usually affect how the thyroid functions.[19]
Understanding Your Metabolic Health With EverlyWell
Thyroid disease is common and can have a significant impact on your metabolism and your health. At EverlyWell, we're here to support you in your healthcare journey. We offer a variety of at-home tests to help you learn more about your health.
If you want to learn more about your thyroid function, consider our Thyroid Test. With a simple finger prick, you can get information on 4 key thyroid biomarkers from the comfort of your own home. Your CLIA-certified results will be delivered a few days after we receive your self-collected sample.
We also offer virtual visits with qualified healthcare professionals if you need help interpreting your thyroid blood test results, or you want more guidance on your thyroid health or any other health condition. Take control of your health with healthcare on your schedule.