Understanding Thyroid Lab Markers
- Gut Honest Truth
- Jan 18, 2024
- 5 min read
Updated: 5 days ago
The answer might surprise you...
Welcome to the Gut Honest Truth blog where a Certified Functional Medicine Practitioner gives you digestible information to tackle your chronic health concerns.
That's me, Katie Morra; and in this blog I'm going to walk you through every thyroid lab marker worth knowing — what each one does, what it means when it's off, and why running only TSH is leaving most people with an incomplete picture.
If you want to learn more about supporting your thyroid health, check out my How to Put the Fire Out on Your Thyroid e-book.
And as always, working one-on-one for your specific needs is always our top recommendation, check out our appointment options to get started with one of our licensed health care professionals today.
What Does the Thyroid Do?
The thyroid is the butterfly-shaped gland in the front of the neck — and it functions as the thermostat of the entire body. It regulates metabolism, heart rate, digestion, muscle control, brain development, mood, body temperature, and bone maintenance.
Over 12% of the US population will develop a thyroid condition in their lifetime. Women are 5-8x more likely than men to be affected, and 1 in 8 women will develop a thyroid condition at some point.
Common symptoms of hypothyroidism or autoimmune thyroiditis:
Unexplained weight gain or inability to lose weight
Cold intolerance
Unexplained fatigue
Dry, coarse skin and hair
Frequent or heavy menstrual cycles
Scalloping of the tongue
Brain fog and forgetfulness
Brittle nails
Anxiety or depression
Outer eyebrow loss
Low sex drive or infertility
Constipation and bloating
Muscle cramps
Acid reflux or hoarse voice
Elevated cholesterol
If several of these resonate and you've been told your thyroid is "fine" — keep reading.
The Problem With Standard Thyroid Testing
Most conventional providers order TSH and maybe T4. If those fall within the normal reference range, you're told your thyroid is fine and sent on your way — even if you feel terrible.
This is like looking at the tip of an iceberg and assuming there's nothing underneath. TSH tells us what the pituitary gland is signaling — not what's actually happening with thyroid hormone production, conversion, or how the immune system is responding.
Here's what a complete thyroid panel should include:
The 6 Thyroid Markers That Tell the Full Story
TSH — Thyroid Stimulating Hormone
The gas gauge. TSH is produced by the pituitary gland and signals the thyroid to produce more hormone. It's a useful starting point — but it tells us nothing about whether the thyroid is actually responding correctly, or whether the hormone being produced is being converted into the active form the body needs.
Conventional range: 0.45–4.5 UIU/ML Optimal range: 1–2 UIU/ML
Free T4 — Thyroxine (Free)
The engine turning on. T4 is the inactive form of thyroid hormone — it must be converted into T3 to be usable. Testing free T4 (rather than total T4) tells us how much unbound, available hormone is in circulation. Bound hormone can't attach to receptor sites and be used by the body.
Conventional range: 0.82–1.77 NG/DL Optimal range: >1.25 NG/DL
Free T3 — Triiodothyronine (Free)
The gas pedal. T3 is the active thyroid hormone — what your cells actually run on. Up to 80% of T3 comes from conversion of T4 in the liver, gut, and kidneys. If you're producing enough T4 but it's not converting properly, you can have all the classic symptoms of hypothyroidism with a normal TSH. You'd never know without testing Free T3.
Conventional range: 2.0–4.4 PG/ML Optimal range: >3.2 PG/ML
Reverse T3 (RT3)
The brakes. RT3 is an inactive form of thyroid hormone that blocks thyroid receptors — essentially competing with T3 and preventing it from doing its job. Elevated RT3 is produced when the body is under chronic stress, fighting inflammation, dealing with gut infections, undereating, or nutrient depleted. It's the body's way of putting the brakes on metabolism during times of perceived threat.
Someone with low T3 and high RT3 will feel profoundly hypothyroid — even with a normal TSH.
Conventional range: 9.2–24.1 NG/DL Optimal range: <15 NG/DL, or a RT3:FT3 ratio of less than 10:1
TPO Antibodies — Anti-Thyroid Peroxidase
Emergency flashers #1. TPO antibodies are produced by the immune system in response to thyroid tissue. Elevated TPO indicates the immune system is attacking the thyroid — the hallmark of Hashimoto's thyroiditis. TPO antibodies are often elevated years before TSH becomes abnormal, which is exactly why they get missed in standard panels.
Conventional range: <34 IU/ML Optimal range: <10 IU/ML
TgAb — Anti-Thyroglobulin Antibodies
Emergency flashers #2. TgAb is a second autoimmune marker, often elevated alongside TPO in Hashimoto's — but some patients have elevated TgAb with normal TPO. Running only one and not the other means missing half the autoimmune picture.
Conventional range: <1.0 IU/ML Optimal range: <1 IU/ML
You can have completely normal TSH, T4, T3, and RT3 — and significantly elevated antibodies — meaning your immune system is actively attacking your thyroid gland and you'd never know without testing all six markers.
Functional vs. Conventional Reference Ranges
A key distinction in functional medicine is the use of optimal ranges rather than standard reference ranges. Standard thyroid lab ranges were established using data that included both healthy individuals and people with thyroid dysfunction — which means "within range" doesn't necessarily mean your thyroid is performing optimally. Functional ranges are narrower and represent where the body is more likely to thrive.
This is one of the most common reasons patients come to us after being told their labs are "normal" — they are normal by conventional standards, but not optimal for how they want to feel.
How Thyroid Hormones Are Made
It all starts in the brain. The body continuously monitors thyroid hormone levels, and in response, the pituitary gland releases TSH to signal the thyroid to produce more hormone (approximately 90% T4, 10% T3).
Once released, T4 must be converted to Free T3 via the 5-deiodinase pathway — primarily in the liver and gut. This is why gut health, stress, and nutrient status matter so much for thyroid function — they directly affect conversion.
Only free hormones — not protein-bound hormones — can actually attach to receptor sites and be used by the body. This is why testing free T4 and free T3 specifically (rather than total levels) is essential to understanding thyroid function accurately.
How to Get a Full Thyroid Panel
Most conventional providers won't order the full panel without advocacy on your part. You can:
Request it directly — ask for TSH, Free T4, Free T3, Reverse T3, TPO, and TgAb
See a functional medicine practitioner who orders comprehensive panels as standard practice
Use ifm.org to find a certified practitioner near you
At GHT, a full thyroid panel is a standard part of our intake process. If your thyroid hasn't been properly evaluated — or if you've been told everything looks fine but you know something is off — we'd love to take a closer look.
For a deeper dive into hypothyroidism and what healing it actually looks like, read: How Do I Start Healing Hypothyroidism?
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