Your thyroid gland is situated in the neck just below the Adam’s apple. The thyroid creates proteins and controls how your body uses energy and your body’s sensitivity to other hormones. Your thyroid produces triiodothyronine, a hormone known as T3. T3, along with T4 (which is also produced in the thyroid), regulates your body’s temperature, metabolism, and heart rate.
Most of the T3 in your body binds to protein. The T3 that does not do so is called free T3, and circulates unbound in your blood. The most common kind of T3 test, known as the T3 total test, measures both kinds of T3 in your blood.
By measuring the T3 in your blood, your doctor may be able to determine if you have a thyroid problem.
Your doctor will typically order a T3 test if he or she suspects a problem with your thyroid. These potential disorders include:
- hyperthyroidism (when your thyroid produces too much thyroid hormone)
- hypopituitarism (when the pituitary gland doesn’t produce normal amounts of pituitary hormones)
- primary or secondary hypothyroidism (when the thyroid doesn’t produce normal amounts of thyroid hormones)
- thyrotoxic periodic paralysis (a disorder that occurs when your thyroid produces high levels of thyroid hormones, which results in muscle weakness)
Your thyroid and the hormones it releases affect much of your body’s function. This means that a thyroid disorder can cause a wide range of symptoms. For example, you might experience mental issues such as anxiety, or physical problems such as constipation and menstrual irregularity.
Other possible symptoms include:
- weakness and fatigue
- difficulty sleeping
- increased sensitivity to heat or cold
- weight loss or gain
- dry or puffy skin
- dry, irritated, puffy, and/or bulging eyes
- hair loss
- hand tremors
- increased heart rate
If you already have confirmation of a thyroid problem, your doctor might choose to order this test to see whether there have been any changes in your condition.
Sometimes, your doctor might also order a T4 test and/or a TSH test. T4, or thyroxine, is another hormone produced by your thyroid. TSH (thyroid stimulating hormone) is the hormone that stimulates your thyroid to produce T3 and T4. Testing the levels of either or both of these other hormones can help give your doctor a more complete picture of what’s going on.
Tell your doctor about all of the medications you are currently taking. Some might affect your T3 test results. If your doctor knows about your medications in advance, he or she can advise you to temporarily stop using them or will consider their effect when interpreting your results.
Medications that can affect your T3 levels include, but are not limited to:
- thyroid-related drugs
- birth control pills and other medications containing hormones such as androgens and estrogens
The T3 test simply involves having your blood drawn. The blood will then be tested in a laboratory.
Your T3 test results will be measured in mcg/dL, which means micrograms per deciliter or nanograms per deciliter. One nanogram [ng] = 0.001 mcrogram [mcg]. Typically, results ranging from 100 to 200 ng/dL are normal.
A normal T3 test result does not necessarily mean that your thyroid is functioning perfectly. Measuring your T4 and TSH can help your doctor figure out if you have a thyroid problem despite a normal T3 result.
Because the thyroid’s functions are complicated, this single test may not give your doctor any definitive answers about what is wrong. However, abnormal results can help point him or her in the right direction. He or she may also choose to perform a T4 or TSH test to gain a clearer picture of your thyroid function.
Abnormally high levels of T3 are common in pregnancy and with liver disease. If your T3 test also measured the free T3 level, your doctor may be able to use this information to rule out these conditions. In other cases, your doctor might order a T3RU test. Much T3 binds to a protein in your blood. The T3RU test measures the availability of this protein in your blood.
High T3 Levels
If you are not pregnant or suffering from liver disease, elevated T3 levels might indicate thyroid issues such as:
- Graves’ disease (an autoimmune disease that causes your thyroid to produce too much hormone)
- painless (silent) thyroiditis
- thyrotoxic periodic paralysis
- toxic nodular goiter (a condition in which an enlarged thyroid gland has malfunctioning thyroid nodules, small rounded growths, that overproduce thyroid hormone)
High levels might also indicate high levels of protein in the blood. In rare cases, these elevated levels could indicate thyroid cancer or thyrotoxicosis.
Low T3 Levels
Abnormally low levels of T3 may indicate the following:
- long-term illness (T3 levels decrease when you are sick. If you’re sick enough to be hospitalized, your T3 levels are likely to be low. This is one reason that doctors do not routinely use the T3 test as a thyroid test, but usually use it along with the T4 and TSH test to get a more complete picture of how your thyroid is working)
When you have your blood drawn, you can expect to have a bit of discomfort during the procedure. You might also experience minor bleeding or bruising afterward. In some cases, you may feel light-headed.
Serious symptoms, such as fainting, infection, excessive bleeding, and inflammation of the vein, are rare.