The consultation goes like this. You've been exhausted for two years. You're cold all the time. Your hair is falling out. You've gained weight despite eating carefully. You can't think clearly after midday. You ask your GP to check your thyroid. The result comes back. TSH 3.8. Normal range 0.5–4.5. Your GP tells you your thyroid is fine.
You leave the surgery no clearer than when you went in.
This scenario plays out thousands of times a week across the UK. And in a significant proportion of those cases, the thyroid is not fine — the test just isn't designed to catch it.
What TSH actually measures
TSH — thyroid stimulating hormone — is produced by the pituitary gland. It's not a thyroid hormone. It's a signal from the brain telling the thyroid to produce more hormone. Think of it as the thermostat rather than the temperature itself.
When the pituitary detects that thyroid hormone levels are low, it releases more TSH to push the thyroid harder. A rising TSH indicates the pituitary is having to work harder — which typically means the thyroid is underperforming. This is why TSH is used as a screening marker.
The problem is that TSH tells you about the signal, not the hormone. It tells you the pituitary is or isn't working hard. It tells you nothing about whether the thyroid is producing adequate Free T4, converting it to active Free T3, or whether autoimmune antibodies are attacking the gland. Those require different tests — which are rarely ordered as part of standard thyroid screening.
The functional optimal versus the conventional range
The conventional upper limit of 4.5 mIU/L is a statistical threshold — it represents the upper boundary of the distribution in the tested population. But significant hypothyroid symptoms — fatigue, cognitive slowing, weight gain, hair loss, cold intolerance — are well documented in the literature at TSH values between 2.5 and 4.5.
The functional optimal of 1.0–2.5 mIU/L reflects the zone where thyroid-dependent processes function optimally — where the pituitary is neither under-stimulating nor over-working to compensate for inadequate thyroid output. A TSH of 3.8 sits squarely within the conventional normal range. It also sits squarely within the zone where hypothyroid symptoms are expected.
The conversion problem: T4 to T3
Even if TSH looks reasonable, the more important question is often what's happening downstream. The thyroid primarily produces T4 — thyroxine — which is a storage hormone. It must be converted to Free T3 — triiodothyronine — before it becomes biologically active. T3 is the hormone that actually enters cells and drives metabolism. Every symptom associated with hypothyroidism is ultimately a T3 problem at the cellular level.
This conversion happens primarily in the liver, gut, and peripheral tissues. It requires selenium, zinc, and adequate iron (specifically ferritin above 60 µg/L). It is directly impaired by elevated cortisol, systemic inflammation (elevated hs-CRP), gut dysbiosis, and caloric restriction.
"I regularly see clients with TSH of 2.8 and Free T4 of 14 — both within conventional normal — and Free T3 of 4.1, which is below the functional optimal of 5.0. Every hypothyroid symptom is present. The standard thyroid screen misses this pattern entirely because Free T3 isn't included."
This pattern — adequate TSH, adequate T4, insufficient T3 — is one of the most common thyroid presentations in functional medicine. The conversion is failing. But if you only test TSH, you never see it.
Hashimoto's: positive antibodies with normal TSH
There is a second way the standard thyroid screen misses genuine thyroid disease: Hashimoto's autoimmune thyroiditis.
Hashimoto's is the most common cause of hypothyroidism in the developed world. It's an autoimmune condition in which the immune system produces antibodies against thyroid tissue — specifically the TPO (thyroid peroxidase) enzyme and thyroglobulin. These antibodies can be elevated — and autoimmune thyroid destruction can be active and progressing — for years before TSH becomes abnormal.
Standard thyroid screening does not include TPO or TG antibodies. A client with TPO antibodies of 180 IU/mL, active Hashimoto's, and a TSH of 2.1 will be told their thyroid is completely normal. They receive no intervention. The autoimmune process continues unchecked.
This matters because Hashimoto's is not solely a thyroid problem — it's an immune dysregulation problem with a thyroid consequence. The intervention is different. Gluten elimination (the most evidence-based first step), gut integrity support, selenium 200mcg daily, and vitamin D optimisation all influence antibody levels and the rate of thyroid destruction. None of this is indicated if the diagnosis isn't made — and the diagnosis is only made if you test for it.
What a complete thyroid assessment looks like
A functional thyroid panel includes: TSH, Free T4, Free T3, reverse T3, TPO antibodies, and TG antibodies. That's six markers. The NHS routinely tests one. Private labs like Medichecks and Thyroid UK offer complete panels — the difference in cost is modest and the difference in clinical information is substantial.
The pattern that changes everything: TSH within conventional normal + Free T3 below 5.0 pmol/L = hypothyroid symptoms explained. The thyroid screen said normal. The Free T3 told the actual story.
If you've been told your thyroid is fine and you still don't feel well, a TSH result alone is not a complete investigation. It's a starting point that the standard system treats as a conclusion.