Factors That Can Affect Thyroid Test Results

Several factors can affect thyroid test results, the blood tests used to measure thyroid-stimulating hormone (TSH), triiodothyronine (T3) and thyroxine (T4), and the levels that are key to your thyroid health.

Some may be short-term fluctuations in thyroid hormone levels that go away when your medication dose changed, or small differences in results when the hour of your test has changed. Other factors that affect thyroid test results may be longer-term, such as pregnancy or a lifestyle factor like smoking that can alter the levels recorded by your test.

This article looks at 10 factors that can affect your thyroid blood test results. It explains why they may alter your results and, in some cases, how you can limit the impact on your thyroid tests.

what affects thyroid test results

Verywell / Emily Roberts

Fasting and Thyroid Test Results

Studies have shown that time of day and fasting can affect your results. If you test in the early morning after overnight fasting, the results may show higher (TSH) levels. If you get your blood test in the afternoon when you have not been fasting, your TSH levels will be lower.

That can be a problem if you have subclinical hypothyroidism. It occurs when you have a mildly elevated TSH and normal free T4, but no symptoms. Free T4 is the active form of T4.

Subclinical hypothyroidism may be missed if you take your test in the afternoon and you have not been fasting. Ask your healthcare provider if you need to fast for your TSH and related thyroid tests.

Stress and Thyroid Test Results

Factors that can affect thyroid test results include mental stress and overall stress on your body, such as sleep deprivation or dieting.

The evidence for the influence of these things is unclear. Still, it is a good idea to have your thyroid levels checked under the same conditions each time. This can help avoid false fluctuations on your test results.

If you are taking thyroid hormone replacement, it's a good idea to get your tests under the same conditions every time. Schedule your TSH tests for around the same time of day. If you fasted for one test, fast for all of them.

Body Weight

Your weight and body mass can affect thyroid function and have an impact on thyroid tests. For example, TSH levels and thyroid hormone levels rise with the release of the hormone leptin, which also rises with obesity.

But many factors are at play in how thyroid function and your weight and body mass actually affect each other. As a general rule, people with an overactive thyroid tend to have lower body weights and those with lower function tend to gain thyroid-related weight.

Body Mass Index and Thyroid Function

Researchers aware of the limitations of body mass index (BMI) in assessing thyroid health are looking at new tests, like waist-to-hip ratio (WHR) and body shape index (ABSI). They're finding more nuances in how weight and body mass affect thyroid function.


The impact of smoking on thyroid function has been known for decades, with higher free triiodothyronine (free T3) levels and lower TSH levels in people who smoke. A recent study of more than 5,700 people in the Netherlands noted modestly higher free T4 levels as well, when smokers were compared with former smokers and people who never did.

Thyroid tests from people who quit smoking show their TSH goes up and the T4 goes down. The reasons why smoking affects thyroid function remain under investigation.

Alcohol Use

Some studies show a lower free T4 level that's seen in people who consume alcohol. Lower free T3 and free T4 levels also have been reported in people following their alcohol withdrawal.

The reasons for thyroid level changes in people who use alcohol remain unclear, but there is some evidence to suggest that alcohol acts on other hormone-producing structures that in turn affect thyroid function, including the hypothalamus (TRH) and pituitary glands (TSH).

It's also possible that elements in the beverages themselves, like resveratrol polyphenols found in red wine, may act on the thyroid and, therefore, affect thyroid test results.

Diet and Thyroid Test Results

Iodine deficiency has long been known to contribute to hypothyroidism, but many other foods may have an impact on TSH and thyroid hormone levels. Those under study include:

  • Soy-based foods
  • Olive oil
  • Broccoli and other brassica vegetables
  • Tea and coffee

Some studies have found diets high in processed foods affect TSH and thyroid hormone levels. A 2021 study of 4,585 people found thyroid-related changes in people with diets high in protein and fats (like bacon and sausage), sugar sources like fruit juice, and grains like those in white bread.

Iodine and Thryoid Tests

You may be asked to avoid iodine in foods like kelp, or in medicine, for about a week before a thyroid scan test. The iodine is a factor that may affect your thyroid test results because of a small amount of radioactive iodine that's used during the scan.

Medication Use and Thyroid Test Results

Certain medications can cause thyroid dysfunction. These drugs can interfere with your thyroid hormone levels or with their action. This may alter TSH because your body may try to compensate for the high or low thyroid hormones.

Thyroid Hormone Function

Some medications can alter thyroid hormone function and test results. These include:

  • Lithobid (lithium)
  • Pacerone (amiodarone)
  • Iodine or kelp supplements
  • Immunomodulating drugs like interferon alpha and interleukin-2
  • Certain chemotherapy drugs
  • Tyrosine kinase inhibitors, like Nexavar (sorafenib) or Sutent (sunitinib)

Thyroid Tests and Birth Control

Estrogens, like those in birth control pills, can cause high levels of T3 and T4. If you are taking estrogens, be sure to have a TSH and free T4 test as part of your thyroid evaluation since they typically will not be affected.

Thyroid Hormone Absorption

Certain supplements and medications can get in the way of your body's ability to absorb thyroid medications. These include:

  • Calcium carbonate
  • Iron sulfate
  • Proton pump inhibitors like Prilosec (omeprazole) and Prevacid (lansoprazole)

These may prevent you from getting enough of your medication into your system. This is why it's typically recommended that you avoid taking any supplements or medications within three to four hours of your thyroid medication.

Thyroid Hormone Measurements

Other medications can interfere with thyroid lab measurements. These drugs do not affect the actual functioning of the thyroid.

A few examples include:

  • Certain nonsteroidal anti-inflammatory drugs (NSAIDs)
  • Certain anticonvulsants
  • Heparin (a blood thinner)
  • Lasix (furosemide)
  • Glucocorticoids (steroids such as cortisone)
  • Isotretinoin

Research has found that the supplement biotin can interfere with thyroid blood test results. If you take biotin, stop two days before your thyroid blood test.

Certain drugs and supplements may interfere with your test results. Some can change thyroid function or get in the way of your body's ability to absorb thyroid medications. Others may interfere with the test itself.

Thyroid Levels During Pregnancy

Thyroid hormone levels change during pregnancy. In fact, the ranges considered normal also change throughout pregnancy. What is considered normal may be slightly different depending on the lab where you get your blood test.

Thyroid levels are measured in milliunits per liter (mIU/L) and picomoles per liter (pmol/L). A mole is a way to measure large amounts of very small things like atoms and molecules. A picomole is one trillionth of a mole.

Normal ranges for non-pregnant people are as follows:

  • TSH: 0.5-4.7 mIU/L
  • Free T4 (FT4): 8.0-18 pmol/L
  • Free T3 (FT3): 2.30-4.2 pmol/L

Normal ranges for pregnant people are different. The American Thyroid Association (ATA) recommends that healthcare providers use these ranges based on trimester when evaluating the thyroid tests of pregnant people.

 1st Trimester  2nd Trimester 3rd Trimester
TSH 0.1 - 2.5 mIU/L 0.2 - 3.0 mIU/L 0.3 - 3.0 mIU/L
FT4 10.30-18.11 pmol/L 10.30-18.15 pmol/L 10.30-17.89 pmol/L 
FT3 3.80-5.81 pmol/L 3.69-5.90 pmol/L 3.67-5.81 pmol/L

Illness and Thyroid Test Results

Certain illnesses can temporarily affect thyroid hormone results. Diarrhea can interfere with your medication absorption and may alter your lab results. Infections or a bout of an inflammatory condition such as lupus can also have an effect.

A syndrome called nonthyroidal illness or sick euthyroid syndrome can also affect thyroid function and test results. This is a serious illness that requires hospitalization.

People with this condition have a normal TSH level with a low T4 and T3 level. These changes in thyroid levels are thought to be protective. For that reason, people with this condition do not receive treatment to correct thyroid levels.

Toxic Exposure

Some heavy metals and other toxins can interfere with thyroid function, which can affect thyroid hormone levels and test results. These environmental exposures include:

  • Arsenic
  • Lead
  • Mercury
  • Nitrate fertilizers
  • Pesticides
  • Perfluoroalkyl substances (PFAS) "forever" chemicals

These toxins can change how thyroid hormones are produced and metabolized (used in the body). However, research on these compounds has led to mixed results, and scientists continue to work to understand how toxins affect thyroid function in both the short and long term.

Different Labs and Test Type

Keep in mind that different kinds of tests, or even the same test done at different laboratories, can yield results that are inconsistent or use different ranges. Talk with your healthcare provider about the kind of blood test or imaging you'll have, and how the pathology (the lab work) or radiology assessment will be done.


Outside factors may affect the results of your thyroid test. This can include whether or not you ate before your test, what time of day you take the test, and things like stress, diet, and sleep.

Some medications and supplements may also affect your results. This may be because they change your thyroid function. It may also be because they reduce your body's ability to absorb thyroid hormone. Other drugs and supplements may just interfere with the test itself.

Pregnant people have different normal thyroid levels than those who are not pregnant. Certain illnesses may also interfere with your results. 

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Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
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Additional Reading

By Mary Shomon
Mary Shomon is a writer and hormonal health and thyroid advocate. She is the author of "The Thyroid Diet Revolution."