Understanding Your Hypothyroidism Test Results
A simple regular blood test is used to guide treatment for hypothyroidism and monitor the condition. Here's what the numbers mean.
By Beth W. Orenstein
Medically Reviewed by Farrokh Sohrabi, MD
If there’s a bright side to hypothyroidism, or an underactive thyroid, it’s that treatment usually just involves taking daily medication, and testing is limited to a simple blood test. Your hypothyroidism test results are your doctor’s blueprint for prescribing the right dose of synthetic thyroid hormone medication and tracking how well it’s working.
Thyroid Hormones: Names and Numbers
The main job of the thyroid gland is to make the hormone thyroxine, also known as T4 because it has four iodine molecules. The thyroid also makes the hormone triiodothyronine, known as T3 because it has three iodine molecules, but in smaller amounts, explains Cathy Doria-Medina, MD, an endocrinologist with HealthCare Partners Medical Group in Torrance, Calif. “The thyroid gland makes mostly T4, [and] the T4 has to be converted to T3 because T3 is the part of thyroxine that actually does the work," she says.
The pituitary gland at the base of the brain controls hormone production in your body. It makes thyroid-stimulating hormone, or TSH, which tells the thyroid gland how much T4 and T3 to produce. The TSH level in your blood reveals how much T4 your pituitary gland is asking your thyroid gland to make. If your TSH levels are abnormally high, it could mean you have an underactive thyroid, or hypothyroidism.
“TSH levels go in the opposite direction of your thyroid hormone,” Dr. Doria-Medina explains. “If you’re making too little thyroid hormone, your TSH will go up. If you’re making too much thyroid hormone, your TSH will go down.”
What’s normal can vary, depending on a number of factors, including the laboratory where your blood test is done, she adds. A normal range for TSH in most laboratories is 0.4 milliunits per liter (mU/L) to 4.0 mU/L.
If your TSH is higher than 4.0 mU/L on repeat tests, you probably have hypothyroidism.
Your doctor may also order a T4 test. Most of the T4 in your blood attaches to a protein, and when it does, it can’t get into your cells. Only T4 that is unattached or “free” can get into your cells to go to work. A blood test can measure how much free T4 is available.
Hypothyroidism Tests: A Measure of Treatment Success
Hypothyroidism is treated with daily medication. Taking synthetic thyroid hormone medication can bring your T4 and TSH levels back to their normal ranges. Once you’re on the right dose, you should have no symptoms.
When you first start taking medication, your doctor will need to monitor your blood to fine- tune the dosage. “Initially you will need to be tested more frequently,” Doria-Medina says. “A person who is newly diagnosed and taking medication for hypothyroidism should be tested every six weeks until the dosage is just right.”
The dose you start with is your doctor’s educated guess about what’s best for you, most likely the lowest dose possible to avoid side effects, which can include a rapid heartbeat and restlessness.
Medication for hypothyroidism is slow-acting, and it can take several weeks for your body to adjust. If your TSH is still high and your symptoms haven’t subsided after six to 10 weeks, your doctor will likely increase the dose, and you’ll need your blood tested again after another six to 10 weeks.
Keeping Hypothyroidism Under Control
Because you’ll need to take thyroid medication every day for the rest of your life, even after the right dose is found, your hormone levels will be monitored regularly to be sure that your treatment is working properly. “Eventually, most people with hypothyroidism can just be seen yearly by their doctor,” Doria-Medina says.
The American Thyroid Association recommends that you keep your TSH within a narrow range of 0.5 to 2.0 mU/L, but don’t be alarmed if your test results vary a little. Some variation is normal because your pituitary gland sends out TSH in pulses, not a steady stream. Also, the time of day you're tested can make a difference. TSH levels are likely to be higher at night and lower during the day.
If you have new or worsening symptoms or your health-status changes — such as if you become pregnant, go through menopause, or are given another medicine that can interfere with your thyroid hormones— you should see your doctor and have your blood tested again, even if it’s ahead of schedule.
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