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Hope you're having a great day! I wanted to send a quick note/reminder about proper thyroid testing for those taking thyroid medications. This is especially important if you recently switched medications or are seeing a new doctor. I've talked about this before, but wanted to give you some support in writing, as well as a reference from a medical journal, in case you ever run into this situation with your doctor and need to advocate for yourself. 

I've been getting a lot of messages lately from people who began to feel hypothyroid after having their thyroid medications reduced based on a faulty lab interpretation. 

One lady began to lose her hair in clumps, and another became nearly bed-ridden! 

What all of these people had in common was that they were feeling great on a regimen of T4 and T3 medications, then found to have a low TSH (suggestive of hyperthyroidism or overdosing) on a monitoring test, which prompted their doctor to reduce their medications. Severe hyperthyroidism can be a life-threatening situation, and doctors worry about even slight hyperthyroidism due to its effects on the heart and bones.

Thyroid hormones are goldilocks hormones, meaning the dose has to be "just right" for us to feel optimal. Doses that are too high or too low can result in symptoms. 

Whenever I see a low TSH number in someone who is taking thyroid medications, my first question is always: "Did you take your thyroid medications before you had your thyroid labs drawn?" 

Depending on the type and timing of medication, a person's TSH number may be inaccurate. 

How to get accurate thyroid lab results when taking medications.

When first initiating treatment with thyroid medications for hypothyroidism, guidelines generally 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 post about interpreting this test HERE). 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.  

    If your doctor will not order these tests for you, you can pay out of pocket and order them yourself thought a company like Direct Labs. To view a list of recommended labs click HERE.    

Slightly different rules apply to the timing of the lab draw based on the type of thyroid medication you are taking...

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 9 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 rollercoaster. 

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 2 hours after your T4 medication is given. 

So let’s say you take your medication at 8am, and have your blood test at 10am- 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.

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 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 really 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 3 days, depending on the person. In some, that’s less than a day for it to start rapidly declining in your body. The T3 levels thus fluctuate in the body throughout the day, and the goal is to have levels within the normal limit for most of the day. 

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 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 5 hours. The TSH level then begins to increase again 5 hours after the dose, and continues to rise until 13 hours after the last dose, after which point it stays stable. Not knowing that the levels will fluctuate, a doctor may see an elevation at one point in the day, and assume that you are overdosed.

Free T3 levels are also affected, increasing after the dose is given and hitting a peak at 4 hours after dose. 

This means that if you were to take your thyroid medication within 5 hours of getting your thyroid function tests done, your lab results may 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 actually be under-dosed. 

Thus, it is usually best to postpone your combination medication until after you get your lab test 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 T3 and T4 testing will come in handy, as well as of course, looking out for symptoms of hypo- or hyperthyroidism. 


1. Saravanan P, Siddique H, Simmons DJ et al. Twenty-four hour hormone profiles of TSH, Free T3 and free T4 in hypothyroid patients on combined T3/T4 therapy. Exp Clin Endocrinol Diabetes 2007; 115(4): 261–267. PubMed Article Number: 17479444

Your Thyroid Pharmacist,

Dr. Izabella Wentz, PharmD, FASCP

If you are looking for more information on putting Hashimoto's into remission, please consider purchasing a copy of my patient guide, Hashimoto's Thyroiditis Lifestyle Interventions for Finding and Treating the Root Cause to help you figure out your underlying triggers.

I also have a website with additional resources for you, including my blog and a list of recommended supplements and clinicians

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