Factors That May Affect Your Thyroid Test Results

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Thyroid blood tests measure thyroid hormone levels. These include thyroid-stimulating hormone (TSH), the hormone that tells your thyroid gland to make thyroid hormone.

Tests also check levels of the thyroid hormones triiodothyronine (T3) and thyroxine (T4), and the proteins that affect thyroid hormone production.

Thyroid blood tests are usually simple and accurate. A few things can affect your results, though.

You may have ongoing fluctuations in thyroid hormone levels. This is a sign you need your medication dose changed.

Other factors may falsely skew your thyroid test results. When this happens, the levels recorded by your test may not indicate there has been a true change in your condition. They can make your thyroid disease seem milder or worse than it really is.

It's a good idea to be aware of the things that can affect your results. That way, you can make sure your thyroid test results are always useful.

This article looks at some of the things that can affect your thyroid blood test results, including fasting, lifestyle choices, and medicines and supplements. It also discusses how pregnancy and illness might affect your test results.

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. This is compared to tests done later in the day with no fasting.

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

This condition is diagnosed and monitored based solely on TSH. T4 and T3 levels are usually normal.

If you get your blood test in the afternoon when you have not been fasting, your TSH levels will be lower. Because of this, your subclinical hypothyroidism may be missed.

Recap

Subclinical hypothyroidism may be missed if you take your test in the afternoon and you have not been fasting.

Lifestyle and Thyroid Test Results

Other factors may affect your thyroid test results, including:

The evidence for the influence of these things is unclear. Still, it is a good idea 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.

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:

Thyroid Hormone Absorption

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

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:

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.

Recap

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:

First Trimester:

  • TSH: 0.1 - 2.5 mIU/L
  • FT4: 10.30-18.11 pmol/L
  • FT3: 3.80-5.81 pmol/L

Second Trimester:

  • TSH: 0.2 - 3.0 mIU/L
  • FT4: 10.30-18.15 pmol/L
  • FT3: 3.69-5.90 pmol/L

Third Trimester:

  • TSH: 0.3 - 3.0 mIU/L
  • FT4: 10.30-17.89 pmol/L
  • FT3: 3.67-5.81 pmol/L

Recap

According to the American Thyroid Association (ATA), normal ranges of TSH and free T4 in pregnant people vary by trimester. The ATA recommends doctors use these ranges when evaluating the thyroid tests of pregnant people.

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.

Summary

Outside factors may affect the results of your thyroid blood 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.

A Word From Verywell

It is important to be consistent about your thyroid tests. Try to take them under similar conditions each time.

If your thyroid blood tests seem "off" for no good reason, a repeat blood test is a reasonable next step.  

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