When and How to Best Do Your Thyroid Blood Tests

Timing, Taking Thyroid Medications Before Testing, and Fasting

The timing and conditions of thyroid blood tests can be important to diagnosis and monitoring. clipart.com

Should you fast before having thyroid tests? Does it matter what time of day—morning or afternoon— your thyroid test panels are run?  Should you take your thyroid medication before or after your thyroid blood testing is done?

If you are a thyroid patient being treated with thyroid hormone replacement medication, these are important questions, given that you are likely to have at least an annual blood test, or likely, even more frequent testing.

More frequent thyroid testing is also more common after your initial diagnosis, when you are changing medications or dosages, or during and after pregnancy.

Fasting Before Thyroid Tests

Usually, doctors tell us that fasting is not necessary for thyroid blood tests.

Interestingly, however, researchers have reported that TSH levels decline in almost all people, by as much as 26 percent on average, compared to early morning, fasting TSH test results. This can result in as many as 6 percent of patients being reclassified from the diagnosis of subclinical hypothyroidism to "normal." Your TSH is likely to be highest, and most reflective of its underlying status, when tested after fasting, in the early morning. 

Timing of Thyroid Tests

Our doctors also often say that it does not matter when your thyroid tests are done, especially the TSH test. Given that TSH tests reflect the impact of several weeks of medication, it's hard to argue that time of day or fasting would have an impact on the testing and results.

However, those patients taking an external source of T3—such as Cytomel, Armour Thyroid, Nature-throid, or a time-released T3 medication—need to be aware of when they've taken their medication and the timing of testing. T3 is active only for hours in the body, while T4, both synthetic and natural, has a longer period of activity.

So, testing of T3 levels shortly after taking a T3 medication may show excessive T3 levels that are not typical or reflective of your usual levels throughout the day. 

Implications for Patients

Essentially, the researchers found that early morning fasting tests might show that you have elevated TSH levels indicative of subclinical hypothyroidism, but when tested later in the day, you may have "normal" TSH levels that are not consistent with subclinical or overt hypothyroidism. 

This is questionable, however, given that researchers have not established that the earlier, fasting level results are any less valid than the later, non-fasting, "normal range" results. Researchers also don't even know how fasting affects thyroid function or TSH values, or the difference of TSH values based on fasting or non-fasting.

In some cases, if you are having your thyroid testing done along with other blood work, such as glucose, insulin, or cholesterol levels were done—you will be asked to fast, and have your test done first thing in the morning.

In other cases, your doctor may not dictate when to have your TSH test, or whether or not too fast.

But the key thing to know: If you test later in the day and you've eaten, you are likely to have a lower TSH level than if you test earlier in the day with fasting. Ideally, you will want to make sure that your TSH testing is done at the same time, and under the same circumstances (i.e., fasting). Comparing morning fasting TSH levels with non-fasting test results from later in the day may not allow for accurate diagnosis and management of your treatment. 

Other Findings Regarding Thyroid Blood Testing

There is other information that supports the idea that patients should not take any thyroid medication prior to running a panel of thyroid tests, and should only be taken after the tests are completed.

According to Thyroid Manager: "Serum T4 concentrations peak two to four hours after an oral dose and remain above normal for approximately six hours in patients receiving daily replacement therapy."

For this reason, thyroid expert Richard Shames, MD has the following recommendation:​ "I absolutely recommend that patients have any morning blood tests evaluating the thyroid before taking any thyroid medication. I have always told my patients to do it this way."

Sources:

Scobbo RR, VonDohlen TW, Hassan M, Islam S. Serum TSH variability in normal individuals: the influence of time of sample collection. W V Med J. 2004;100:138-142.)

Thyroid Manager, Section 9.8, Treatment of Hypothyroidism. Online.

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