Is There a Right Time or Way to Have a Thyroid Test?

Timing, fasting, and medication use can all affect results

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Thyroid testing can be a tricky thing. While the tests offer a high degree of accuracy, we know that hormone levels can vary from one day to the next and even from one part of a day to the next. While these fluctuations won't make a difference in most cases, there are times when they will.

Take, for example, the case where your T3 and T4 are normal but your TSH is suddenly low (suggesting you are either hyperthyroid or taking too much medicine).

Your doctor may rightly want to adjust your dosage, but, in doing so, you start to feel lethargic, cold, and may even experience a little hair loss (all signs that you are either hypothyroid or getting too little medicine).

If something like this happens, it doesn't necessarily mean that the labs got it wrong. Thyroid hormones are often referred to as "Goldilocks hormones" because it can take time to get them just right. The same applies to how lab tests are interpreted. Based on when the test is taken, when you've taken your drugs, and whether you've eaten or not, the readings may one day suggest you are too low and the next day suggest you are too high.

Getting it right, therefore, requires that you to understand why these fluctuations occur and what you can do to obtain the most accurate results possible.

How Fasting Affects Thyroid Tests

Most guidelines will suggest that fasting is not necessary when performing a thyroid blood test.

This can be a confusing issue since studies have regularly shown that early morning fasting translates to higher TSH levels—as much as 26 percent higher—compared to those taken later in the day with no fasting.

What this means is that as many as one in 15 people may be reclassified from subclinical hypothyroidism in the morning to normal in the afternoon.

How Timing and Medication Use Affects Thyroid Tests

The guidelines also suggest that it doesn't really matter when during the day you get your tests done, and, in large part, that's true. TSH tests, in particular, are meant to evaluate the impact of treatment over the course of weeks and, for the most part, hormone levels tend to remain relatively stable.

The same doesn't apply to T3 levels or the drugs use to treat them. While T4 medications such as Synthroid and Levoxyl have a half-life of five to nine days (meaning it takes five to nine days to clear the 50 percent of the drug from your body), T3 drugs like Cytomel (liothyronine sodium) have a half-life of as little as one day. This translates to greater fluctuation in T3 levels with peaks typically occurring within four hours of taking the drug. 

What this means is any T3 test performed right after taking at T3 drug may reflect higher values than actually exist.

Even with T4 drugs, which maintain a far steadier state, a peak can occur within two to four hours and remain elevated for up to six hours after taking the dose.

Putting the Findings to Practical Use

While it is clear that fluctuations can occur for any number of reasons, the actual implication of these fluctuations remains unclear.

In terms of fasting, for example, there is still no consensus as to whether earlier fasting results are any less valid than the later non-fasting results. In fact, we're still not even sure how fasting affects thyroid function or to what degree it may significant. All we just know is that it can happen.

With that being said, the findings do provide us a means to ensure more accurate results when monitoring thyroid function. Consider the following when next getting tested:

  • Always get tested at the same time of day, in the same manner (fasting/non-fasting), and at the same lab to ensure consistency and a more accurate reflection of your condition. This is especially true if you are managing well on your current medications.
  • If you are testing later in the day with normal values but aren't experiencing the benefits of replacement therapy, speak with your doctor about testing at different times of the day to see if and how values fluctuate.
  • If possible, take your hormone replacement medications at least six hours before a test or wait until after you've had the test to take them.
  • Take your medications consistently as prescribed. While you may be tempted to self-dose because it "makes you feel better," the tests will not provide as clear a reflection of your health and may make treating the disease all the more difficult if problems arise.
  • If you're not managing well on your current medications, see your doctor but continue treatment until tests can be performed and evaluated.

Source:

Koulouri, O. and Gurnell, M. "How to interpret thyroid function tests." Clinical Medicine. June 1, 2013; 13(3):282-286.

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