Optimal Time and Conditions for Thyroid Blood Tests

Test Timing, Should You Take Medication Beforehand, and Should You Fast?

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 your thyroid tests are done?  These are important questions for thyroid patients, who typically have blood tests at least annually, and often more frequently, especially after initial diagnosis, when changing medications, or while pregnant. 

Fasting Before Thyroid Tests

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

Interestingly, however, research by Scobbo et. al. in 2004 showed that TSH tests declined in 97 of 100 of the people studied -- by an average of around 26 percent -- when compared to the early morning, fasting TSH test results. This resulted in as many as 6% of patients being reclassified from the diagnosis of subclinical hypothyroidism to "normal." This means that your TSH is likely to be highest when tested after fasting, in the early morning. 

The researchers concluded that the diagnosis of subclinical hypothyroidism should not be made only on a fasting TSH measurement.

Timing of Thyroid Tests

Our doctors also often say that it does not matter when your thyroid tests are done -- especially the Thyroid Stimulating Hormone (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-thyroid, 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 elevated TSH levels indicative of subclinical hypothyroidism, but when tested later in the day, these patients had "normal" TSH levels and were not hypothyroid.

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 bloodwork -- 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.

Other Findings

Other information actually supports the idea that patients should not take any thyroid medication the day of testing, until after tests are completed.

The Thyroid Manager online textbook mentions that

Serum T4 concentrations peak 2 to 4 hours after an oral dose and remain above normal for approximately 6 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.

More Information

For more help understanding thyroid blood tests, see:

What Do Your Thyroid Blood Tests Really Mean

All About Thyroid Blood Tests

Key Thyroid Function Tests: Laboratory Values and Interpretation

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.)

Shames, Richard, MD. Email interview, June 30, 2012.

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

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