What You Need To Know About Doing Thyroid Blood Tests

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Originally published on 30th May 2017
Last updated on 2nd November 2018

Common questions about thyroid blood tests for those with medical conditions such as a thyroid disorder include:

  • Do I take my thyroid medication as normal before the blood draw?
  • Do you need to fast for thyroid blood work?
  • How often should it be done?
  • Does it need to be in the morning?
  • Can it be done in the afternoon?
  • Is fasting required for a TSH test?
  • What should be tested?

A Thyroid Blood Testing Kit

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

When To Take Medication

In regards to T4-only medication such as Levothyroxine and Synthroid, they only have 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.

Therefore, whether you take T4-only medication right before your blood test or blood sample is taken, or haven’t taken it for up to two days beforehand, your TSH levels and Free T3 levels should still be the same, but free T4 may well show a peak two hours after taking T4 medications.

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 of 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 eighteen hours.

Straight after taking a T3 containing medication, the TSH level begins to fall and then stays suppressed for as long as five hours. Free T3 levels also increases after taking T3 medication and hit a peak after three to four hours. This means that if you were to take your T3 containing thyroid medication within five hours of getting your thyroid tests done, your test results may imply that you are overmedicated when you’re not, or even that your levels are within range or optimal, when you’re actually under-medicated. So it can affect your ability to get an accurate result and adjust your dosage accurately.

Therefore, you’re best to hold off taking this medication until after the blood draw. I take my thyroid medication with me so that I can take it straight after the blood test, otherwise I start to feel unwell by midday.

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

If you’re also one of the thyroid patients who take the supplement biotin, it is also worth knowing that it can cause falsely elevated thyroid levels on test results, making you look overmedicated or hyperthyroid when you’re not. Therefore, it is advised to not take it for at least 48 hours before a thyroid blood test.


So, do I have to fast for TSH blood work? Do thyroid blood tests require fasting?

In terms of fasting, most doctors tell thyroid patients that it’s not necessary to fast before a blood test. However, researchers have found that after eating, our TSH level becomes suppressed. This means that a high TSH could instead look much lower after eating, and borderline levels no longer borderline. 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, this could result in patients having their thyroid medication wrongly altered, or even being told that their ‘borderline’ hypothyroidism is now ‘normal’, resulting in some thyroid patients being inadequately treated for their thyroid condition.  All because they ate before their test.

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 9am is preferable. This is because thyroid hormone levels have a circadian rhythm with a peak at night, so Dr. Geracioti suggests that blood tests for hypothyroidism be done before 9am in order to not miss subclinical hypothyroidism and have as accurate results as possible.

How Often?

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 for hypothyroidism (when the thyroid gland located in the neck doesn’t produce enough hormone), 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. When your thyroid gland isn’t working properly, thus leading to hypothyroidism, it’s incredibly important to correct the low levels of thyroid hormone with frequent and thorough testing.

What Should Be Tested?

Full Thyroid Panel (also known as thyroid function tests) is required to get the most comprehensive and accurate look at how you’re doing, as TSH alone and even TSH and Free T4 test without the other components of the thyroid panel, don’t give the full picture. Free T3 and Thyroid Antibodies TpoAB and TgAB are important to monitor too. Many thyroid patients find that whilst their TSH is OK, their Free T3 and Free T4, or even Reverse T3 isn’t optimal and so they still feel unwell.


If you’re looking at ordering some of your own tests online, please see some helpful information here. You can also view all tests for TrueHealthLabs European patients here – https://www.truehealthlabs.com/Thyroid-Europe-s/1880.htm and all tests for US residents here – https://www.truehealthlabs.com/Thyroid-s/1840.htm

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

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Written by Rachel, The Invisible Hypothyroidism

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Rachel Hill, Thyroid Patient Advocate, blogger and author, has Six 2018 WEGO Health Award Nominations. She is a highly ranked writer appearing in the Top Hypothyroidism Websites and Top Thyroid Websites and has worked with The National Academy of Hypothyroidism, The BBC, The Mighty, Yahoo, MSN and ThyroidChange, to name just a few. She is well-recognised as a useful contributor to the thyroid community and also contributed the foreword to Emily Kyle’s The 30-Minute Thyroid Cookbook.

4 thoughts on “What You Need To Know About Doing Thyroid Blood Tests

  1. I have recently changed from Canadian ERFA “natural” thyroid sold as “the Armour thyroid that FOREST LABS MANUFACTURED. I have been battling many illnesses in the last few years. I have changed to Thyrovanz.. I am wondering if this product has the actual RAW T3 ? IF NOT it is just more fake advertising. I need the T3!! Where can I safely purchase a product with natural T3 if Thurman turns out to be unreliable?

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