why TSH alone says nothing about thyroid function
In Australia, to investigate thyroid function most doctors begin by testing TSH (Thyroid Stimulating Hormone).
TSH is a hormone produced by your pituitary gland (in your brain). It’s role is to tell your thyroid to produce more thyroid hormone when needed. If TSH is low or normal, most doctors assume your pituitary is happy, which means the thyroid is happy, which means your metabolism must be fine.
But there is a fatal flaw in this logic, because
TSH only tells us what the brain sees. It tells us absolutely nothing about what is happening inside your actual tissue cells.
Once your thyroid gland releases thyroid hormone (mostly in the inactive form called T4), that hormone has to travel through your bloodstream to your liver, gut and peripheral tissues to be converted into the active form, T3.
T3 is like the metabolic spark plug. It enters your cells, fires up your mitochondria and dictates how quickly you burn energy, regulate temperature and maintain body composition.
When this signal is blocked:
If your body senses that you are under chronic stress, under-eating, micronutrient deficient, over-training or dealing with systemic inflammation, it goes into survival mode. It decides that running a roaring, fast metabolism is a luxury it can’t afford.
To adapt, your body shifts its enzymatic pathways. Instead of converting inactive T4 into active T3, it activates an enzyme called Type III deiodinase. This enzyme converts your thyroid hormone into a mirror-image molecule called Reverse T3.
What standard blood tests don’t see:
Reverse T3 physically sits inside your cellular thyroid receptors. It acts like a broken key jammed in a lock, blocking active T3 from getting in. So, your brain thinks everything is fine (TSH stays completely normal), but your actual cells are functionally starving for thyroid hormone. Your metabolism stalls.
This is why you can do everything right with your diet and exercise and not see your body composition budge an inch. Your cells are literally locked out of their primary power supply.
How do we get the full picture?
To get the real story, you need test a Complete Thyroid Panel, which includes:
TSH: The brain's demand signal
Free T4: The raw, storage hormone your thyroid produces
Free T3: The active, usable hormone that actually fires up your cells and burns fat
Reverse T3: The metabolic brake pedal that blocks your cells when you are stressed, inflamed, malnourished or over-training
Thyroid Antibodies (TPO & TG): To rule out underlying auto-immune involvement
Without looking at these functional pieces, you're only seeing a fraction of the picture. If you’re not confident that your doctor knows what to test or how to interpret the results, it’s very okay to seek a second opinion. Or reach out and I’d be happy to guide you through this.
With love xx