The Gap Series · Essay 7 · Thyroid · Autoimmunity
TSH is a pituitary hormone. It measures what the pituitary thinks about thyroid function — not what the thyroid is actually doing, not whether T4 is converting to the active T3, and not whether the immune system is quietly attacking thyroid tissue. A single TSH result cannot answer these questions. But in most GP surgeries, it is the only question being asked.
This is part of The Gap — a series of clinical essays on what medicine misses and what functional investigation offers instead. Earlier essays cover reference ranges, why women are sicker and diagnosed later, and stress as biochemistry.
I want to describe a patient pattern so common in my practice that I have stopped thinking of it as unusual and started thinking of it as a near-default presentation for women in their late thirties and forties.
She is tired. Not occasionally tired — persistently, unrelentingly tired in a way that doesn't respond to sleep, that she manages rather than resolves, that has been present for so long she has stopped mentioning it to doctors because the response is always the same. Her weight has shifted — not dramatically, but enough that her body feels different from how it used to, and different in ways that don't respond to the things that used to work. Her hair is coming out in the shower. She's cold when other people aren't. Her thinking is slower — not obviously, not in ways other people notice, but she notices. There's a flatness to her mood that doesn't feel like depression but doesn't feel like herself either.
She has had blood tests. TSH was normal. She has been told her thyroid is fine.
In a meaningful proportion of these cases, the thyroid is not fine. The investigation was simply inadequate to detect what's actually happening.
Thyroid stimulating hormone is produced by the pituitary gland. It is the signal the pituitary sends to the thyroid when it detects that circulating thyroid hormone levels are too low — a request for more production. When TSH rises, it means the pituitary is asking louder. When TSH falls, it means the pituitary is satisfied.
This is useful information. But it is information about the pituitary's assessment of thyroid hormone levels — not a direct measurement of thyroid hormone levels themselves, not a measurement of whether thyroid hormone is being converted into its active form at the tissue level, and not any kind of measurement of whether the immune system is attacking the thyroid gland.
Relying on TSH alone to assess thyroid function is like assessing a central heating system by reading the thermostat. The thermostat tells you what the control system thinks is happening. It doesn't tell you whether the boiler is actually firing efficiently, whether the heat is reaching the radiators, or whether there's a slow leak somewhere that's undermining the whole system.
A complete thyroid picture requires more than one number.
The markers that together give a clinically useful picture of thyroid function:
Reverse T3 is genuinely difficult to obtain through standard NHS or even most private testing in the UK — it requires specialist referral or a specific private panel. This is a real limitation. Clinically, however, the picture can often be inferred: a patient with normal TSH, normal free T4, low-normal free T3, a history of chronic stress or prolonged caloric restriction, and a full symptom burden consistent with hypothyroidism is exhibiting the pattern that elevated reverse T3 would explain — even without the number. The intervention (addressing the underlying stressor, supporting conversion) is the same regardless.
The step that gets missed most often in the thyroid story is not production — it's conversion.
The thyroid produces primarily T4 — a prohormone that is metabolically inactive until converted to T3. This conversion happens mainly in the liver (approximately 60%) and the gut (approximately 20%), with the remainder occurring in peripheral tissues. It requires specific cofactors — selenium is the most important, but zinc, iron, and iodine all play roles — and it is inhibited by several factors that are extremely common in people who feel unwell.
Chronic cortisol elevation — the physiological signature of sustained stress — directly inhibits the deiodinase enzymes responsible for T4-to-T3 conversion and promotes production of reverse T3 instead. This is one of the primary mechanisms by which chronic stress produces hypothyroid-like symptoms in people with a normal TSH: the pituitary is satisfied because T4 is adequate, but the active T3 that tissues actually need is being under-produced and competed for by an inactive isomer.
Gut dysbiosis impairs conversion because approximately 20% of T4-to-T3 conversion depends on bacterial enzymes in the intestinal tract. A significantly dysbiotic gut microbiome — a finding that is common in people presenting with multiple chronic symptoms — reduces this conversion capacity meaningfully. This is one of the reasons that gut investigation often turns out to be relevant in thyroid presentations that aren't obviously gut presentations.
Selenium deficiency directly impairs the deiodinase enzymes. The UK population is chronically selenium insufficient — the soils are depleted and the dietary sources (Brazil nuts, seafood, organ meats) are not reliably consumed. This is a correctable nutritional factor that is almost never considered in conventional thyroid management.
Iron deficiency impairs thyroid peroxidase, the enzyme the thyroid uses to produce hormone in the first place. Low ferritin — extremely common in women of reproductive age, and often not adequately treated even when identified — compromises thyroid function upstream. The fatigue of low ferritin and the fatigue of suboptimal thyroid function are clinically similar. They frequently coexist. They share a root cause in dietary inadequacy and blood loss. And neither is detected by a TSH.
Hashimoto's thyroiditis is an autoimmune condition in which the immune system produces antibodies against thyroid tissue — specifically against thyroid peroxidase (TPO) and thyroglobulin (TgAb). It is the most common cause of hypothyroidism in the developed world and the most common autoimmune disease overall.
The critical clinical point about Hashimoto's is that antibody elevation precedes TSH abnormality — often by years. During this window, the immune system is actively attacking thyroid tissue, inflammatory damage is accumulating, and the patient is experiencing symptoms. But because TSH is within the conventional reference range, no investigation is triggered, no diagnosis is made, and no intervention is offered.
TPO antibodies are not routinely checked unless TSH is already abnormal. This means that the investigation protocol is structured to find Hashimoto's only after it has progressed far enough to significantly impair thyroid output — by which point a meaningful amount of thyroid tissue may already have been damaged.
Checking TPO antibodies (and TgAb) in any woman presenting with fatigue, unexplained weight change, cognitive slowing, hair loss, cold intolerance, or mood disturbance — regardless of TSH — is, in my view, the minimum adequate thyroid investigation. It is not what happens routinely.
Hypothyroidism — whether from Hashimoto's, poor conversion, or suboptimal production — produces a characteristic symptom constellation. The challenge is that each individual symptom is non-specific. The pattern is what's diagnostic.
Basal body temperature — the body's resting temperature taken first thing in the morning before getting up — is a simple and underused clinical indicator. A consistent BBT below 36.5°C in the context of the above symptoms suggests suboptimal thyroid function even when TSH is normal. Broda Barnes, who pioneered this approach in the mid-twentieth century, was dismissed by mainstream medicine for decades. The clinical observation remains valid.
Hashimoto's is an autoimmune condition. This means the primary problem is not the thyroid — it is immune dysregulation that has targeted the thyroid as its expression. Managing it effectively requires addressing the immune terrain, not just replacing the hormone output.
The factors that drive autoimmune thyroid disease overlap substantially with the factors that drive autoimmune disease generally:
Intestinal permeability. The gut barrier is the largest interface between the immune system and the external environment. When intestinal permeability is increased — allowing bacterial products, food antigens, and other compounds to translocate across the gut wall — immune activation follows. The molecular mimicry hypothesis suggests that immune responses triggered against certain food proteins (most notably gliadin, the protein fraction of gluten) can cross-react with thyroid tissue antigens due to structural similarities. The evidence for a gluten-Hashimoto's connection is not definitive but is substantial enough to warrant clinical consideration — particularly given that intestinal permeability is a finding we can investigate and address.
Vitamin D insufficiency. Vitamin D is not simply a bone mineral — it is a potent immunomodulator with specific relevance to autoimmune disease. Low vitamin D is consistently associated with higher rates of autoimmune conditions including Hashimoto's. In Scotland, where adequate sun exposure is seasonal at best, functional vitamin D insufficiency is extremely common. Optimising vitamin D (to 100–150 nmol/L rather than merely above the 50 nmol/L NHS threshold) is a foundational intervention in autoimmune thyroid disease.
Selenium. The thyroid contains the highest concentration of selenium per gram of any organ in the body. Selenium is essential for deiodinase enzyme function (T4-to-T3 conversion), for glutathione peroxidase activity in the thyroid (protecting against oxidative damage), and for the immune modulation that prevents excessive autoimmune attack. Selenium supplementation has been shown in randomised controlled trials to reduce TPO antibody levels — one of the few nutritional interventions with this level of evidence in thyroid autoimmunity.
Chronic infection and immune burden. Epstein-Barr virus (EBV) has been associated with Hashimoto's onset — the virus appears to be able to trigger or amplify the autoimmune response against thyroid tissue in susceptible individuals. Chronic immune burden from any source — gut dysbiosis, recurrent infections, chronic viral reactivation — creates an immune environment more permissive to autoimmune activity.
Chronic stress and cortisol dysregulation. The HPA axis and thyroid axis are in continuous crosstalk. Chronic cortisol elevation suppresses T4-to-T3 conversion, impairs TSH secretion, and creates an immune environment — with a shift toward Th2 dominance and reduced regulatory T cell activity — that is permissive to autoimmune thyroid disease. Managing Hashimoto's without addressing the stress physiology is managing one branch of a system while ignoring the root.
I am not making the argument that everyone with a normal TSH and thyroid symptoms has Hashimoto's or significant thyroid dysfunction. I am making the argument that a single TSH result is insufficient to rule it out.
The investigation that would actually answer the question is not complicated or expensive. Free T4, free T3, TPO antibodies, and TgAb — alongside the broader clinical picture — give a meaningful functional thyroid assessment. Our Randox blood chemistry panel includes TSH, free T3, free T4, and TPO antibodies as standard, alongside the iron panel (ferritin, serum iron, transferrin saturation), vitamin D, selenium where indicated, and the inflammatory markers that contextualise the whole picture.
That's the difference between a test and an investigation.
A TSH tells you what the pituitary thinks. A proper thyroid investigation tells you what the thyroid is doing, whether it's converting hormone effectively, and whether the immune system is involved. These are different questions — and only one of them is currently being asked.
The woman I described at the beginning of this post — tired, cold, losing hair, told her thyroid is normal — deserves the second set of questions. Not because the first question was wrong to ask, but because it wasn't enough.
In thirty-seven years of clinical practice, I have seen this pattern resolve — sometimes remarkably — when the investigation was adequate and the intervention was targeted accordingly. Selenium repletion, vitamin D optimisation, gut investigation and repair, stress physiology addressed via DUTCH testing, cortisol pattern managed, conversion supported nutritionally. These are not heroic interventions. They are the logical consequences of a clinical investigation that took the question seriously.
The thyroid is not the last thing to investigate. It should be among the first. And investigating it properly takes more than one number.
The Randox blood chemistry panel includes: TSH · Free T3 · Free T4 · TPO Antibodies · Ferritin · Full iron panel · Vitamin D · CRP · Full blood count · and 130+ additional markers in functional reference ranges.
For a comprehensive hormonal picture including the cortisol-thyroid relationship, the DUTCH Plus maps HPA axis function, cortisol pattern, and the stress hormone picture that sits underneath thyroid conversion.
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