T3 is a vital hormone; it controls your metabolism, body temperature, and heart rate. But there can be too much of a good thing. High T3 levels may increase your risk for pregnancy complications, liver disease, and may even reduce your sex drive. Find out the common causes of high T3 levels and the possible health risks.
The other main hormone made by the thyroid is T4, which acts as a precursor to T3. Cells can convert T4 into T3 as needed. While T4 does have some biological activity, T3 is about four times more active and does most of the work in the body .
High T3 levels are usually a sign of hyperthyroidism, a condition where the thyroid produces too much thyroid hormone. In this article, we’ll list out some of the major causes of high T3 and the health risks associated with them .
You can also learn much more about T3 and T4 in our thyroid hormone article here.
In the section below, we’ll list out some potential causes of high T3 levels. This is not a comprehensive list. If you suspect you may have an imbalance in thyroid hormones, speak to your doctor.
Graves’ disease is an autoimmune disorder where the body mistakenly produces antibodies that can bind to the TSH receptor. This binding will stimulate the TSH receptor, leading to an overproduction of thyroid hormones. In the U.S., Graves’ disease is the most common cause of hyperthyroidism, accounting for about 50-80% of cases .
The symptoms of Graves’ disease are similar to other hyperthyroidism conditions and can include hand tremors, trouble sleeping, intolerance to heat, and weight loss. Graves’ disease can also cause Graves’ ophthalmopathy, an eye disease that causes eye-bulging and lid retraction .
Treatment of this condition usually involves antithyroid drugs (like methimazole), radioactive iodine, or surgery to remove all or parts of the thyroid. Eye symptoms are usually treated with eye drops or surgery in severe cases .
Thyroid tumors (also known as thyroid adenomas) are benign tumors of the thyroid gland. They are often described as “hot” or “cold”. A “cold” thyroid tumor is inactive and does not produce any thyroid hormone. On the other hand, a “hot” tumor is active and produces T3 and T4, which can lead to hyperthyroidism .
In some parts of the world, thyroid tumors account for almost 50% of all cases of hyperthyroidism. However, if the tumor is not causing any negative effects, the doctor may decide to simply monitor the condition. Otherwise, the tumor can be surgically removed .
A goiter simply means an enlarged thyroid gland. The most common cause of goiter is iodine deficiency, but many other factors like inflammation can contribute to this enlargement .
Most of the time, a goiter is benign and does not cause any ill effects. In some cases, small nodules can form on the enlarged thyroid which can produce T3 and T4. The release of thyroid hormones from these nodules can lead to hyperthyroidism. Treatment usually involves antithyroid drugs, radioactive iodine, or surgery .
Thyroid hormone levels naturally increase during pregnancy because the mother must also supply T3 and T4 to the developing baby. The body normally does this by increasing levels of hCG, a hormone produced by the placenta that can bind to TSH receptors, which produces more thyroid hormones [7, 8].
T3 and T4 levels increase significantly in the 1st trimester of pregnancy and gradually decline during the 2nd and 3rd trimesters. Levels should return to normal within 1-2 months after giving birth [7, 8].
On rare occasions, thyroid levels can increase too much during pregnancy, causing hyperthyroidism. This usually happens when the mother also has another condition that affects the thyroid, such as Graves’ disease [7, 8].
In many parts of the world, people do not receive enough iodine in their diet, which can lead to high rates of hypothyroidism. That is why iodine is commonly added to salt, making iodized salt, to help improve intake .
However, it is possible to consume too much iodine, which can lead to hyperthyroidism in rare cases. The recommended upper limit of iodine is 1100 micrograms. Excessive amounts of iodine supplements or high-iodine foods (like seaweed or fish) can potentially lead to hyperthyroidism. Those with thyroid nodules are more at risk, according to research [10, 11].
Research suggests that exposure to even low levels of cadmium can cause the thyroid gland to become hyperactive and secrete more T3 and T4. Cadmium exposure is also linked to thyroid tumors, which further increase the levels of thyroid hormones [12, 13].
Several medications can increase the level of thyroid hormones in the blood.
The most obvious example would be thyroid medications that are used to treat low thyroid levels (hypothyroidism). For example, desiccated thyroid (Armour Thyroid) is a drug that contains T3 and T4 extracted from animals. Taking an excessive amount of these drugs can cause thyroid hormone levels to become too high [14, 15].
- Amiodarone, used to treat irregular heartbeats
- Interferon alpha, used to treat various cancers
- Human growth hormones (somatotropin)
In the section below, we’ll discuss some potential health risks of high T3 levels in the blood. This is not a comprehensive list. Having high T3 levels does not necessarily mean these health risks will occur. If you have abnormal thyroid hormone levels, speak to your doctor about how they may affect you.
High T3 levels in the blood may lead to symptoms of hyperthyroidism. Symptoms can vary greatly depending on age, the severity of the disease, and the presence of other conditions. The most common symptoms of hyperthyroidism include :
Hyperthyroidism can also sometimes lead to thyroid storm, a life-threatening condition that is usually triggered by another serious illness, such as fever, infection, or a heart attack. Those with uncontrolled hyperthyroidism and those who abruptly stop their antithyroid medications are more at risk .
Thyroid storm is essentially a severe form of hyperthyroidism and shares many of the same symptoms. However, a thyroid storm can also cause fever, confusion, heart attack, and jaundice (yellowing of the skin and eyes). This condition requires immediate medical attention .
High levels of thyroid hormones can be dangerous for both the mother and baby. For example, women with uncontrolled hyperthyroidism are about 9 times more likely to have a low birth weight pregnancy and are almost 17 times more likely to give birth prematurely [19, 8].
- Preeclampsia, a form of high blood pressure that can occur in pregnant women
- Heart failure
- Thyroid storm
- Birth defects
- Hyperthyroidism in the baby
- Low birth weight
Proper control of thyroid hormones is critical during pregnancy. Dosing requirements for thyroid medications often change while pregnant. Always speak with your doctor if you plan to become pregnant or are pregnant and have high thyroid hormone levels .
Metabolic syndrome refers to a collection of disorders that includes obesity, high blood pressure, high blood sugar, high blood triglycerides (fats), and low HDL (good cholesterol). People with three or more of these conditions are considered to have metabolic syndrome .
One observational study of 245 people who do not have hyperthyroidism suggests that those with higher free T3 levels (3.37–4.12 pg/mL) are about twice as likely to have metabolic syndrome than those with lower levels (2.32–2.96 pg/mL). Interestingly enough, everyone in the study had free T3 levels that are considered normal (2.0-4.4 pg/mL) .
Thyroid hormones play a major role in metabolism, which includes regulating cholesterol levels. An observational study looked at the connection between thyroid hormones and cholesterol levels in 296 elderly patients who have diabetes, but normal thyroid function .
Non-alcoholic fatty liver disease (NAFLD) describes the buildup of fat in the liver that is not caused by alcohol consumption. This condition increases the risk of other liver diseases, including cirrhosis, liver cancer, and liver failure .
According to an observational study of 2,576 people with normal thyroid function, high free T3 levels are linked to higher rates of NAFLD. This association was not seen with free T4 or TSH .
Men may be at risk as well. Two studies with over 3,000 men each found that hyperthyroidism may increase the risk of severe erectile dysfunction .
High T3 levels are most commonly caused by thyroid disorders like Graves’ disease, goiter, and thyroid tumors. Health risks of high T3 include pregnancy complications, high cholesterol, liver disease, and sexual dysfunction. Lowering T3 levels usually involves treating the underlying condition, which requires medical attention.
This post is part of a series about thyroid hormones. Read the other parts to learn about: