Lab Draw Timing While on Thyroid Medications for Accurate Results

Recommended Lab Tests

When first initiating treatment with thyroid medications for hypothyroidism, guidelines recommend testing thyroid function about 4-6 weeks after the start of the treatment to determine if the dose of medication is correct. It may be prudent to run these tests sooner and more often if the person was severely hypothyroid, hyperthyroid, or experiencing symptoms of either.

In addition to looking out for symptoms of hypothyroidism or hyperthyroidism, I also recommend lab testing to monitor. There are three main thyroid tests I recommend to measure your response to medications:

• TSH – This is a pituitary hormone that responds to low/high amounts of circulating thyroid hormone. In advanced cases of Hashimoto’s and primary hypothyroidism, this lab test will be elevated, (read a post about interpreting this thyroid TSH test). In the case of Graves’ disease, the TSH will be low. People with Hashimoto’s and central hypothyroidism may have a normal reading on this test.
• Free T3 & Free T4 – These tests measure the levels of active thyroid hormone circulating in the body. When these levels are low, but your TSH tests in the normal range, this may lead your physician to suspect a rare type of hypothyroidism, known as central hypothyroidism.

T4-Only Medications like Synthroid, Levoxyl, Tirosint, and Levothyroxine

T4 has a five to nine-day long half-life, which means that once you have become stable on it, it will take as many as nine days for 50% of the dose to clear the body. T4 has a slow and steady release and won’t produce many peaks or valleys that can make a person feel like they are on a thyroid roller coaster.

Whether you take T4 right before your lab tests or haven’t taken it for up to 48 hours, your TSH value should be the same. Thus, you will get an accurate representation of your TSH value whether or not you take your T4 medication before a lab test.

Same goes for free T3 levels when you take a T4 medication. The free T3 level will also be relatively constant.

Levels of free T4, however, will show a peak two hours after your T4 medication is given.

So let’s say you take your T4 medication at 8 am and have your blood test at 10 am – your thyroid labs may show that your T4 is falsely elevated, and this may result in your doctor lowering your medication, when in reality, your T4 levels may be within range for the rest of the day, with the exception of that 2 hour, post dose peak.

In most cases, taking a T4 medication the morning before your lab test will not be an issue, as most doctors adjust the dosages according to the TSH, which stays stable after T4 dosing. However, to get a reading of your T4 levels that is reflective of most of the day, you would want to postpone your T4 medication until after the lab test.

Combination Medication

If you’re taking a combination medication that contains T3, like Armour, Nature-Throid, WP Thyroid, compounded T4/T3, or the medication Cytomel (liothyronine), the timing of your tests does matter and may make a huge difference in getting you on an appropriate medication dose. This is because T3 has a half-life of only 18 hours to three days, depending on the person. In some, that’s less than a day for it to start rapidly declining in your body.

When scientists monitored the levels of TSH, free T3, and free T4 in people with hypothyroidism who were taking combination thyroid medications over a 24 hour period, they found that TSH levels may be falsely suppressed for 5 hours after taking a T3 containing medication. Right after taking a T3 containing medication, the TSH level begins to drop and stays suppressed for five hours. The TSH level then begins to increase again five hours after the dose and continues to rise until 13 hours after the last dose, after which point it stays stable. So in order to get an accurate representation of your stable TSH on a T3 containing medication, you would need to postpone your T3 containing medications until after your test—or wait 13 hours after testing!

Free T3 levels are also affected by a recent dose of T3 containing medications. T3 levels increase after the dose is given, and hit a peak at four hours after the dose.

This means that if you were to take your thyroid medication before getting your thyroid function tests done, your lab results might show that you are overdosed, even when you may be accurately dosed, or they may show your labs to be within normal limits when you may be truly under-dosed.

Thus, it is usually best to postpone your combination medication until after you get your lab tests done. I recommend getting your thyroid function tests done first thing in the morning, bringing your medications with you, and taking them right after you have your thyroid function tests to ensure that you get accurate test results.

**Please note, the half-life of the medications may vary per person, so some people may have a falsely suppressed TSH, even at the euthyroid state, when taking T3 containing medications.

This is when free T3 and T4 testing will come in handy, as well as of course, looking out for symptoms of hypo- or hyperthyroidism.

You may read the entire original post from Dr Izabella Wentz, the Thyroid Pharmacist here:
https://thyroidpharmacist.com/articles/how-to-get-accurate-lab-tests-when-taking-thyroid-medications/