Factors That May Affect Your Thyroid Test Results

Fasting, Medication, Pregnancy, and Severe Illness May Be the Culprits

Thyroid blood tests are generally pretty straightforward and accurate, but sometimes certain factors can affect a person's individual results. With that, in order to obtain the most precise results for yourself (or your loved one), it's a good idea to be aware of these factors.

what affects thyroid test results
​Illustration by Emily Roberts, Verywell

How Fasting Affects Thyroid Tests

While fasting is not considered necessary when performing a thyroid blood test, studies have shown that early morning fasting translates to higher TSH levels, compared to those taken later in the day with no fasting.

This fasting/non-fasting variation can be especially problematic for people with subclinical hypothyroidism (defined by a mildly elevated TSH and normal free T4) since the diagnosis and monitoring of this condition relies solely on the TSH value (since the T4 is normal).

In other words, a diagnosis of subclinical hypothyroidism may theoretically be missed if the TSH value is slightly lower due to a non-fasting, afternoon blood draw.


If you are taking thyroid hormone replacement, it's probably sensible to schedule your TSH blood draws around the same time of day and in the same manner (fasting/non-fasting). 

How Medication Use Affects Thyroid Tests

Certain medications can cause thyroid dysfunction, meaning a person can develop hypo- or hyperthyroidism, as a result of taking the drug. Just a few examples of such medications include:

  • Lithium
  • Amiodarone
  • Iodine or kelp supplements
  • Immunomodulating drugs like interferon alpha, interleukin-2, and Lemtrada (alemtuzumab)
  • Tyrosine kinase inhibitors, like Nexavar (sorafenib) or Sutent (sunitinib)

Certain drugs can also interfere with thyroid hormone replacement absorption in the gut, like calcium carbonate, iron sulfate, and the proton pump inhibitors Prilosec (omeprazole) and Prevacid (lansoprazole).

Other medications can interfere with thyroid laboratory measurements, but not with the actual functioning of the thyroid. A few examples of these medications include:

  • Certain nonsteroidal anti-inflammatories (NSAIDs)
  • Certain anti-convulsants
  • Heparin (a blood thinner)
  • Lasix (furosemide)
  • Glucocorticoids 

Lastly, research has found that the supplement biotin (taken in doses of five to 10 milligrams) can interfere with the measurement of thyroid blood tests. Therefore, it's recommended that people who take biotin hold it for two days prior to having their thyroid blood tests.

How Pregnancy Affects Thyroid Tests 

Due to the increased metabolic needs during pregnancy, there are changes that occur in a woman's thyroid physiology.

The American Thyroid Association recommends that a doctor use trimester-specific reference ranges for TSH and free T4 ranges during pregnancy.

Why Pregnant Women Need to Insist on a Thyroid Test

How Illness Affects Thyroid Tests

If a person is seriously ill, especially if they are hospitalized in an intensive care unit, their illness may affect their thyroid function. In this syndrome, TSH is usually on the low normal side with a low T4, free T4, and T3 level.

Due to this phenomenon, called nonthyroidal illness or "sick euthyroid syndrome," experts generally recommend that doctors do not measure thyroid blood tests in critically ill patients unless there is a strong suspicion for thyroid disease.

A Word From Verywell

In the end, in order to be proactive in your thyroid care, be sure to inform your doctor if any of these factors are present, especially if you are pregnant.

Keep in mind, though, if your thyroid blood tests seem "off" for no good reason, a repeat blood test is always easy to do, and a reasonable next step.  

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