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Updated July 30, 2015.

Usually, our doctors tell us that fasting is not necessary for thyroid blood tests, nor does it matter when tests are done -- particularly TSH tests. Only 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, as T3 is active only for hours in the body, while T4, both synthetic and natural, has a much longer period of activity.


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.

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 26.39% -- when compared to 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."

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

Implications for Patients

Essentially, these researchers retested patients later, when they could get a normal range result and declare the patients to be free of hypothyroidism.

Doing so, however -- retesting a patient without fasting, later in the day, in order to see if an elevated TSH level will then drop down to be "borderline normal" -- is questionable medicine, at best.

It's particularly questionable, 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 why TSH values would be different 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 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 to fast. But be aware that 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 expertRichard 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

<sub>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.</sub>