Why Your Thyroid Tests Look Normal But You Still Feel Awful

Thyroid, Women's Health

The Reverse T3 Story Your GP Probably Hasn’t Told You

If you’ve been told your thyroid is “fine” but you still feel exhausted, foggy, cold all the time, constipated, or like your metabolism has fallen off a cliff, this one is for you.

It’s a story I hear in clinic almost every week. A client comes in with a long list of classic hypothyroid symptoms. They’ve already been to their GP, had their TSH checked, and been reassured that “everything looks normal.” They walk away feeling dismissed, confused, and still very much unwell.

Here’s the part that often gets missed: a normal TSH does not necessarily mean your thyroid is functioning well. There is another piece of the puzzle that standard testing in Australia rarely looks at, and it could be the very thing driving your symptoms. It’s called reverse T3, or rT3.

Most GPs do not routinely test for reverse T3, and many will not test free T3 either. That means you can have a thyroid problem that is invisible on a standard pathology request. To understand why, it helps to walk through how thyroid hormone is actually made and activated in the body.

A Quick Look at How Your Thyroid Actually Works

Your thyroid is a small, butterfly-shaped gland at the front of your neck. It might be small, but it has an enormous influence on how you feel. It governs your metabolism, energy, body temperature, mood, digestion, sleep, hair, skin, and weight.

Producing thyroid hormone is not a single event. It’s a multi-step process that depends on the right raw materials, the right cofactors (specific vitamins and minerals), and well-functioning organs beyond the thyroid itself. When any one piece of this system is under-supplied, symptoms can appear even when standard tests look normal.

Step 1: Making T4, the Storage Hormone

Your thyroid mostly produces T4 (thyroxine). T4 is the storage form of thyroid hormone. It circulates in your bloodstream until your body converts it into the active form, T3, where and when it’s needed.

To make T4, your thyroid combines the amino acid tyrosine with four iodine atoms. Sounds simple enough, but several specific nutrients have to be present for that process to run smoothly.

The Cofactors Your Thyroid Needs to Make T4

  • Iodine: the core building block. Each T4 molecule contains four iodine atoms.
  • Tyrosine: the amino acid backbone of the hormone.
  • Iron: required by the enzyme thyroid peroxidase, which attaches iodine to tyrosine.
  • Selenium: protects the thyroid from oxidative damage produced during hormone synthesis.
  • Zinc: supports TSH production from the pituitary, which signals the thyroid to make hormone.
  • Vitamin A: helps regulate TSH and supports iodine uptake by the thyroid.
  • Vitamin D: supports immune regulation around the thyroid.
  • B vitamins (especially B2 and B3): fuel the cellular energy needed for hormone production.
  • Vitamin C and vitamin E: antioxidant support during the iodination process.
  • Magnesium: involved in countless enzymatic reactions, including thyroid hormone synthesis.

If any of these are running low, your thyroid output can suffer. And here’s the thing: nutrient deficiencies are far more common than most people realise, particularly in women who menstruate, anyone on long-term medications, those with gut issues, and people under chronic stress.

Step 2: Converting T4 into T3, the Active Hormone

Here’s where things get really interesting, and where most thyroid problems actually live.

T3 (triiodothyronine) is the active form of thyroid hormone. It’s roughly three to four times more potent than T4 and is what actually binds to receptors inside your cells to drive metabolism, energy, and gene expression.

Your thyroid only produces a small amount of T3 directly. Around 80 per cent of your active T3 is created elsewhere in the body by removing one iodine atom from T4. This conversion is performed by a family of enzymes called deiodinases.

Where Conversion Happens

  • Liver (around 60 per cent): the primary site of T4-to-T3 conversion. A sluggish liver, fatty liver, poor bile flow, or chronic alcohol use can all reduce conversion.
  • Gut (around 20 per cent): healthy gut bacteria assist with conversion. Dysbiosis, inflammation, leaky gut, and chronic constipation can all impair this. This is one of the big reasons I focus so heavily on gut health with my thyroid clients.
  • Other tissues: smaller amounts of conversion happen in the kidneys, muscles, brain, and elsewhere, where local conversion ensures cells get the active hormone they need.

The Cofactors Your Body Needs to Convert T4 into T3

If any of these nutrients are low, your body may struggle to make enough active T3 even when T4 levels look fine on paper:

  • Selenium: the most critical cofactor for conversion. Deiodinase enzymes are selenoproteins, meaning selenium is literally built into their structure.
  • Zinc: required for deiodinase activity and for T3 binding to its receptors. Zinc deficiency is a common, often overlooked driver of low T3.
  • Iron (and ferritin): low ferritin is strongly linked to reduced T3 levels.
  • Vitamin A: supports T3 binding to its cellular receptors. Without enough vitamin A, even adequate T3 may not exert its full effect.
  • Vitamin D: supports thyroid receptor function so cells can respond to T3.
  • Vitamins B6 and B12: support energy and methylation pathways involved in conversion.
  • Magnesium: needed for enzyme activity and ATP production.
  • Glutathione: your body’s master antioxidant, which protects deiodinase enzymes and supports liver detoxification.

And Now, the Plot Twist: Reverse T3

Not all T4 becomes active T3. Your body has a built-in safety mechanism that lets it convert T4 into reverse T3 (rT3) instead. Reverse T3 is essentially an inactive form of thyroid hormone that acts as a brake on metabolism.

Reverse T3 has the same chemical components as T3, but the iodine is removed from a different position on the molecule. This means rT3 cannot activate thyroid receptors. Worse, it can occupy receptor sites and block T3 from working, slowing your metabolism even further.

Why Does the Body Make rT3?

Reverse T3 is a protective response. When your body senses stress, illness, or a need to conserve energy, it shifts conversion away from active T3 and toward inactive rT3. Metabolism is dialled down so the body can preserve resources during difficult times.

This was a brilliant adaptation for our ancestors during periods of famine, infection, or injury. The trouble is that modern life provides plenty of chronic, low-grade stressors that can keep this brake firmly engaged for years.

Common Triggers for High rT3

  • Chronic stress: persistently elevated cortisol shifts conversion towards rT3.
  • Acute or chronic illness: infections, surgery, trauma, and inflammatory conditions all raise rT3.
  • Crash dieting or chronic under-eating: your body interprets low food intake as famine.
  • Nutrient deficiencies: particularly low selenium, zinc, iron, or B12.
  • Liver or gut dysfunction: impaired detoxification and dysbiosis disrupt the normal conversion pathway.
  • Certain medications: including beta-blockers, some antidepressants, and steroids.
  • Heavy metals and environmental toxins: mercury, lead, and others interfere with deiodinase enzymes.
  • Poor blood sugar regulation: insulin resistance and frequent blood sugar swings encourage rT3 production.

Why This Matters for You

This is the heart of the issue. You can have a perfectly normal TSH and a normal T4, and still have a body that is producing too much rT3 and not enough T3. Standard GP testing simply doesn’t capture this picture.

If you’ve been told everything is fine but you’re still experiencing:

  • Fatigue that doesn’t lift no matter how much you sleep
  • Feeling cold when others are comfortable
  • Stubborn weight that won’t shift with diet and exercise
  • Brain fog and poor concentration
  • Hair loss, dry skin, or brittle nails
  • Constipation or sluggish digestion
  • Low mood or feeling flat
  • Heavy or irregular periods in women

It is absolutely worth digging deeper. The full thyroid panel I run with clients includes TSH, free T4, free T3, reverse T3, and thyroid antibodies (TPO and Tg). The ratio of free T3 to reverse T3 is often more revealing than TSH on its own.

Putting It All Together

Healthy thyroid function relies on so much more than just the thyroid gland itself. It depends on a well-nourished thyroid with all the right cofactors, a healthy liver and gut to convert T4 into active T3, well-managed stress and inflammation to keep reverse T3 in check, and strong cellular receptors so your tissues can actually use the hormone you produce.

When we look at the thyroid through this wider lens, real solutions become possible. Not just managing symptoms, but addressing the root causes.

Ready to Get to the Bottom of Your Symptoms?

If any of this sounds like you, I’d love to help. In clinic I take a thorough history, order comprehensive thyroid testing (including the markers your GP probably hasn’t run), and look at the whole picture: your nutrient status, gut health, liver function, stress, and hormones.

You don’t have to keep feeling this way, and you don’t have to settle for “everything looks normal” when your body is clearly telling you otherwise. Book an appointment and let’s find out what’s really going on.

→ Book your appointment today ←

Disclaimer

This article is for educational purposes only and is not intended to diagnose, treat, or replace medical advice. Always consult a qualified healthcare practitioner before making changes to medication, supplementation, or your diet, particularly if you have a diagnosed thyroid condition.