When you understand the thyroid lab tests that are out there and which ones you should order, you'll save money, time, and - literally - blood by asking for the right tests and knowing what they mean.
We've talked about signs of a thyroid issue and why you should still be skeptical of your lab results if they come back as 'normal' while you're experiencing symptoms. Most importantly, we covered the difference between the conventional (wide) lab reference range vs. the functional (narrower) lab reference range. You want functional ranges - this is the range for optimal health.
In this article:
Why your doctor might be ordering the wrong thyroid tests
Which thyroid lab tests to ask for and what the results could mean
How to order your own thyroid labs if your doctor won't work with you
How often you should test your thyroid
Your doc could be ordering the wrong thyroid tests.
Ever thought about what it's like to go through med school? Even completing my training in functional medicine coaching & nutrition therapy was intense and there was a hoard of information to memorize.
That said, very few general physicians & family doctors can be specialists, though endocrinologists are somewhat better. Either way, if they're conventionally-trained, they may not be ordering you the lab tests you need to get a full picture of what's going on with your thyroid.
Many doctors will order a TSH and if you're lucky, T4 as well. That's not nearly enough to draw a conclusion about your thyroid health.
So which thyroid tests should you absolutely get?
These are the thyroid lab tests (blood) you should always get, in order to make sure you've got a good picture. Many labs/offices have a thyroid panel, but don't assume - make sure the panel includes each of these results and add on those that aren't included.
Below you'll also find a quick summary of what it means, roughly, to have a high, low, or normal result within functional ranges. This information is provided only for educational purposes, as I'm not a doctor. Thyroid labs tell a unique story in combination with each other, so you should always work with a qualified functional medicine doctor to determine what the collective group of your results means.
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TSH (Functional range: .9 - 2.0 IU/mL)
Short for Thyroid Stimulating Hormone. It is a signaling chemical created by the pituitary gland that tells the thyroid how much hormone to make.
TSH is like the supervisor on an assembly line where the thyroid is the only worker: When your body needs more thyroid hormone so the supervisor (TSH elevates) yells louder, "make more hormone"! When your body has enough/too much thyroid hormone, the supervisor speaks softly or says nothing (TSH is normal or low), and less hormone is sent from the thyroid.
⬆️ TSH high: Thyroid is working overtime and the body is signaling for more active thyroid hormone. This can be a sign of hypothyroidism.
⬇️ TSH low: Too much thyroid hormone. You might be overmedicated or overproducing thyroid hormones.
↔️ TSH normal: Your thyroid is functioning well without any added stress.
Free T4 (Functional range: 1.4 - 1.8 ng/dL)
T4, thyroxine, is an inactive or stored form of thyroid hormone, so it's ineffective without being converted to its active form. Free T4 measures the thyroxine in your system that is not bonded to any other molecules or proteins. It hasn't been used up yet, and that's what we want to know - how much thyroid hormone is available to do its job.
⬆️ FT4 high: Excessive thyroid hormone - overactive thyroid or possibly overmedicated.
⬇️ FT4 low: The thyroid isn't making enough hormone; hypothyroidism/low thyroid function.
↔️ FT4 normal: Your thyroid is creating enough hormone without any added stress.
Free T3 (Functional range: 3.00 - 4.50 pg/mL)
T3, triiodithyronine, is the active form of thyroid hormone, and has been converted by losing an iodine atom from its original T4 form. Free T3 measures similarly to Free T4 in that it tells us how much is still available for use in your body. If it's bound to other proteins, it's already been used up and can't be dedicated to metabolic processes (like creating energy, regulating substances in and out of cells, etc.).
⬆️ FT3 high: Excessive thyroid hormone - overactive thyroid or possibly overmedicated.
⬇️ FT3 low: The thyroid isn't converting enough T4 hormone to the active form; hypothyroidism/low thyroid function. There are many causes.
↔️ FT3 normal: Your thyroid is converting enough hormone without any added stress.
Reverse T3 (Functional range: 9 - 35 ng/dL)
Reverse T3 (RT3) is a look-alike hormone T3, active thyroid hormone, but it does not actually activate any thyroid receptors. It is also generated from T4. Think of it like a key that fits in a lock but can't unlock the door. RT3 also becomes elevated under stress, which slows down the thyroid, metabolism, and you as a whole. You don't need to test RT3 it every time, but if given the opportunity and the price is right, get it. Test RT3 as a baseline if this is your first thyroid test or first in a while.
⬆️ RT3 high: Excessive conversion of T4 to inactive RT3. Might be reflection of your adrenal health and how your body is managing stressors, since RT3 conversion is elevated under stress.
↔️ RT3 normal: Your thyroid is properly converting T4 to T3 and RT3 in optimal ratios. There is always some level of RT3 in your body.
TG Antibodies (Functional range: <1 IU/mL)
Thyroglobulin (TG) is a protein made by the thyroid itself. It's the base building block upon which iodine atoms are attached to create thyroid hormones. Antibodies to thyroglobulin attack and destroy the thyroid's ability to create thyroid hormones. A positive/elevated result is also an indicator for Hashimoto's.
⬆️ TG Ab high: The immune system is attacking/destroying your thyroid, which could indicate a positive Hashimoto's diagnosis. Hashimoto's is the autoimmune form hypothyroidism that affects up to 95% of hypothyroid patients.
TPO Antibodies (Functional range: <9 IU/mL)
Thyroid peroxidase (TPO) is an enzyme in the thyroid that attaches iodine atoms to thyroglobulin to create T4 and T3 hormones. TPO antibodies (also called Anti-TPO) are immune proteins that attack this substance and the thyroid itself. This lab result tells you the severity or strength of the autoimmune attack on your thyroid. A positive/elevated result is also an indicator for Hashimoto's.
⬆️ TPO Ab high: The immune system is attacking/destroying your thyroid, which could indicate a positive Hashimoto's diagnosis. Hashimoto's is the autoimmune form hypothyroidism that affects up to 95% of hypothyroid patients.