The thyroid gland is located in the neck, just below the Adam’s apple. The thyroid creates hormones and controls how your body uses energy and your body’s sensitivity to other hormones.
The thyroid produces a hormone called triiodothyronine, known as T3. It also produces a hormone called thyroxine, known as T4. Together, these hormones regulate your body’s temperature, metabolism, and heart rate.
Most of the T3 in your body binds to protein. The T3 that doesn’t bind to protein 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 they suspect a problem with your thyroid.
Potential thyroid disorders include:
- hyperthyroidism: when your thyroid produces too much thyroid hormone
- hypopituitarism: when your pituitary gland doesn’t produce normal amounts of pituitary hormones
- primary or secondary hypothyroidism: when your thyroid doesn’t produce normal amounts of thyroid hormones
- thyrotoxic periodic paralysis: when your thyroid produces high levels of thyroid hormones, resulting in muscle weakness
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, or bulging eyes
- hair loss
- hand tremors
- increased heart rate
If you already have confirmation of a thyroid problem, your doctor might use a T3 test to see whether there have been any changes in your condition.
Sometimes, your doctor might also order a T4 test or a TSH test. TSH, or 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.
It’s important to tell your doctor about all of the medications you’re currently taking, as some may affect your T3 test results. If your doctor knows about your medications in advance, they can advise you to temporarily stop using them or consider their effect when interpreting your results.
Some medications that can affect your T3 levels include:
- thyroid-related drugs
- birth control pills or 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.
Typically, normal results range from 100 to 200 nanograms per deciliter (ng/dL).
A normal T3 test result doesn’t 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 them in the right direction. Your doctor 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 pregnant women and those with liver disease. If your T3 test also measured the free T3 level, your doctor may be able to rule out these conditions.
High T3 levels
If you’re not pregnant or suffering from liver disease, elevated T3 levels might indicate thyroid issues, such as:
- Graves’ disease
- painless (silent) thyroiditis
- thyrotoxic periodic paralysis
- toxic nodular goiter
High T3 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 hypothyroidism or starvation. It could also indicate that you have a long-term illness since T3 levels decrease when you’re 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. Instead, they often 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 may also have minor bleeding or bruising afterward. In some cases, you may feel light-headed.
Serious symptoms, though rare, can include fainting, infection, excessive bleeding, and inflammation of the vein.