What You Need To Know About Doing Thyroid Blood Tests

Common questions about thyroid blood tests include: Do I take my thyroid medication as normal before the blood draw? How often should it be done? Do I need to fast? Does it need to be in the morning? Can it be done in the afternoon? 

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Probably the most important thing to be aware of, is whether you should take your thyroid medication before or after having your testing done.

In regards to T4-only meds such as Levothyroxine and Synthroid, T4 has a half-life of around five to nine days, which means that once you’ve become stable on a dose, it takes around a week for half of that dose to clear the body and blood levels to reflect this. This is why, when some people decide to stop taking it, they feel fine for the first week or so. T4 has a slow, steady release.

Therefore, whether you take T4-only medication right before your blood test, or haven’t taken it for up to forty-eight hours beforehand, your TSH levels and Free T3 levels should still be the same. So you should get an accurate reading of your TSH and Free T3 levels whether or not you take T4 medication before a blood test. Levels of free T4, however, will show a peak two hours after your T4 medication is given.

According to Thyroid Manager: “Serum T4 (Free 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.”

So, if you were to take your T4-only thyroid medication before a thyroid blood draw, your Free T4 levels could come out elevated, leading to your doctor lowering the medication dosage, when you don’t actually need it lowering. However, in most cases, taking T4-only medication the morning before your test will not be an issue, as most doctors tend to adjust dosages according to TSH only, which shouldn’t be affected (although this is incorrect. It should be adjusted based on the Free T3 and T4 levels). But to get a reading of your Free T4 level that is reflective of most of the day (and something your doctor should also be testing, alongside TSH) you will need to hold fire on taking your medication until after the lab test.

Now, if you’re taking a thyroid medication that contains T3, such as NDT or T3 synthetic Liothyronine, it’s important to be aware that T3 has a half-life of around 18 hours.

Straight after taking a T3 containing medication, the TSH level begins to fall and stays suppressed for 5 hours. The Free T3 level increases after taking the medication and hits a peak after three to four hours. This means that if you were to take your T3 containing thyroid medication within 5 hours of getting your thyroid tests done, your test results may imply that you are overdosed, even when you’re accurately dosed. They could also suggest your levels are within normal ranges or optimal, when you’re actually under medicated. So it can affect your ability to get an accurate result and adjust dosage properly.

Therefore, you’re best to hold off taking this medication until after the blood draw. Instead, take it with you and take it immediately after the test so it starts kicking in ASAP.

The bottom line is: you’ll get a more accurate result if you take any thyroid medications after the blood test. 

Now, in terms of fasting, most doctors tell thyroid patients that it’s not necessary to fast before a blood test. However, researchers have reported that when we eat before a thyroid blood test draw, our TSH levels can decline by as much as 26%, compared to early morning fasting TSH test results. As so many doctors use the TSH level to decide if a patient is adequately treated, or in need of more or less thyroid medication (which is inaccurate anyway, without the Free T3 and Free T4 levels), this could result in patients having their medication wrongly altered, or even being told that their ‘borderline‘ hypothyroidism is now ‘normal’, resulting in some thyroid patients being inadequately treated for their underactive thyroid.

Therefore, your TSH level is likely to be at its highest and most reflective of its underlying status, when tested after fasting, in the early morning. 

Another thing to keep in mind is the time at which your blood is drawn for thyroid testing.

 Each time you have your thyroid tests done, you should aim for it to always be done at the same time, and under the same circumstances (i.e. fasting), so they’re as accurate and comparable as possible, when looking at them altogether, to see what’s going on. Given that you shouldn’t take your medication until after the draw, as early as possible in the morning and before 10am is preferable.

In terms of how often you should be having your thyroid tests conducted, once every six to twelve months is standard if your levels are optimal and you feel well. Every two months is more common if you’re still adjusting dosage and having symptoms. Many patients need their dosage altering as the weather gets colder, and again when it warms up, to reflect an increase or decrease in demand for thyroid hormone, due to external temperatures.

When first starting thyroid medication, most doctors recommend testing thyroid levels again about four to six weeks after the start of the treatment, to determine if the dose of medication is correct, but waiting eight weeks can allow the medication to finish building in the body and supply a more accurate reading.

You can click on the hyperlinks in the above post to learn more and see references to information give. Also see these links:

https://www.verywell.com/optimum-time-and-conditions-for-thyroid-blood-tests-3232911

https://thyroidpharmacist.com/articles/how-to-get-accurate-lab-tests-when-taking-thyroid-medications/

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I run a Facebook group, called Thyroid Family: Hypothyroidism Advice & Support Group. This group is for underactive thyroid/hypothyroidism patients only, and not medical professionals or anyone else. If you have any questions on living with hypothyroidism, or want some support, help or advice, please join us. 

I also run a group for the spouses, partners and other halves of hypothyroid patients, called Hypothyroid Patients Other Halves – Support & Advice Group. This is for the other halves only and not patients. 

-Rachel

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