Thyroid level testing is a fact of life if you’re being treated for hypothyroidism — and accurate testing is key. But many things can throw off your test results. Knowing what those things might be is paramount to keeping them from affecting your tests and, ultimately, your health.
The medication your doctor prescribes for hypothyroidism replaces the thyroid hormone that your body doesn’t make enough of. The goal of taking the medication, a synthetic hormone, is to keep your thyroid function within the same range as that of people without thyroid problems, according to the American Thyroid Association (ATA).
In a majority of patients, says the ATA, this treatment eliminates symptoms of hypothyroidism — such as fatigue, dry skin, hair loss, weight gain, and sensitivity to cold. Generally, the goal of treatment is to keep thyroid-stimulating hormone (TSH) levels within a range of 0.5 to 2.5 milliunits per liter (mU/L), according to the ATA. This level may not be appropriate for everyone, so talk to your doctor about what your personal goal is.
To monitor your condition and determine whether your medication dosage needs to be adjusted, your doctor will perform regular blood tests to check your TSH levels. Results may vary, though, from one test to another. Some slight variation in test results is normal and nothing to worry about, according to UCLA Health, but more significant variations could indicate the need for a dosage change.
Here’s what you should know to ensure that you get the most accurate thyroid test results.
TSH Testing Variables
These are 10 factors that can affect your thyroid levels:
You get tested at different times of the day. Thyroid hormone levels tend to go up at night and down during the day, according to the ATA. However, this variation is very slight, says Terry F. Davies, MD, a professor of medicine, endocrinology, diabetes, and bone disease at the Icahn School of Medicine at Mount Sinai in New York City.
You go to different labs for testing. Results may vary from lab to lab, but even if you go to the same lab, your results can vary from test to test, notes UCLA Health. The lab may get one reading on one occasion and a slightly higher or lower reading the next.
You switch brands. A number of companies manufacture thyroid hormone, and their products are slightly different from each other. Your body can react differently to the different brands. To avoid fluctuations in TSH levels, the ATA recommends that you stick with the brand or generic manufacturer that you know works for you.
You skip pills. To get the most out of your treatment, it’s important to take your thyroid medication as prescribed. Although the medication you take — T4 (thyroxine), the main hormone made by the thyroid — stays in your blood a long time, missing a few days in a row, or consistently missing now and then over time, could affect your test results, says Norma Lopez, MD, an associate professor of endocrinology at Loyola Medicine in Maywood, Illinois.
You take the wrong dosage. Say you are scheduled to take 700 micrograms (mcg) a week (100 mcg once a day), but you miss two pills and end up taking only 500 mcg. Your thyroid blood tests will reflect the lower dosage. The same goes if you take too much. “Taking even an extra half pill for several weeks can affect your thyroid levels,” says Dr. Lopez.
You’re inconsistent. One day you take your pill on an empty stomach, say, and the next you take it with a meal. The best time of day to take thyroid medication is generally when you wake up in the morning and can take it on an empty stomach, according to the ATA. That’s because food can affect the way the hormone is absorbed. But the most important thing is to be consistent and take your medication at the same time and in the same way every day, the ATA says. If you always take your pill with meals, you may need a higher dosage than if you always take it without having eaten.
You become pregnant. You still have to take your thyroid medication if you become pregnant. In fact, you might even need more because both you and your developing baby need it. “As soon as we find out a woman is pregnant, we increase her thyroid dosage,” Lopez says. Like pregnancy, menopause can also affect your hormone levels. “Some women need less thyroid medication in menopause,” Lopez says, “but not always.”
You take other medications. Some medications can interfere with your body’s ability to process or absorb thyroid hormone. For example, starting or stopping oral contraceptives can make a difference in the dose of thyroid hormone you require, Dr. Davies says. Also, medications that contain iron or calcium can affect absorption, he says. Antidepressants, cholesterol-lowering drugs, and corticosteroids can affect your thyroid levels, too, according to the ATA. So can certain herbs and supplements, so tell your doctor if you add any of these to your daily regimen. Biotin supplements, for example, can interfere with thyroid function tests by showing falsely low TSH levels. The ATA recommends you stop taking biotin for at least two days before a thyroid test to avoid misleading results.
You get sick. “If you become very sick, your TSH can be suppressed,” Davies says. Being ill can put stress on your endocrine system, so your doctor may need to review your medication while you’re sick.
You change your diet. Certain foods, like walnuts, soy, and foods high in fiber, can interfere with the absorption of your thyroid medication.
Thyroid Testing Reminders
If your doctor decides to adjust your medication dose based on your symptoms and test results, “you’ll need to be tested every six to eight weeks to be sure you’re getting your thyroid levels to where they need to be,” Lopez says. “If you’ve been on a stable dosage for a year or two, testing every six months to a year is appropriate.”
Talk to your doctor if you experience changes in symptoms. Your medication may have to be adjusted, depending on what your test results show.
Additional reporting by Julie Stewart.