Your Thyroid Is “Normal”: Why TSH Alone Tells You Nothing
TSH 2.8 mIU/L. Reference range 0.4-4.0. Normal. Case closed.
Except it isn’t. That number is the pituitary gland’s opinion about circulating thyroid hormone levels. It says nothing about what is happening at the cellular level — whether T3, the active thyroid hormone, is reaching the mitochondria in your muscles and brain and actually driving the metabolic processes it’s supposed to drive. TSH is a signal about a signal. It is the thermostat’s reading of the room temperature, not a measurement of whether the radiators are actually warming anyone.
For a healthy person, TSH is a reasonable proxy. The system works: the pituitary senses thyroid hormone levels, adjusts TSH accordingly, and the thyroid responds. For a patient with chronic illness, this feedback loop can decouple in at least three places — each producing a state where the patient is cellularly hypothyroid with a “normal” TSH.
1 The three ways thyroid function breaks without TSH noticing
Central hypothyroidism. The pituitary itself is underperforming. It doesn’t produce enough TSH to drive the thyroid adequately. The thyroid, receiving insufficient stimulation, produces less T4 and T3 — but the TSH reads “normal” because the pituitary’s idea of “normal” has been recalibrated downward. This occurs in any condition that affects pituitary function: chronic illness, head trauma, pituitary tumours, and — relevantly — chronic HPA axis suppression, which is documented in ME/CFS. The hypothalamic-pituitary-thyroid axis doesn’t exist in isolation; it cross-talks with the HPA axis, and suppression of one can drag the other down (Mancini et al. 2018).
Impaired T4 to T3 conversion. The thyroid gland primarily produces T4 (thyroxine), a prohormone. T4 must be converted to T3 (triiodothyronine) by deiodinase enzymes in peripheral tissues for it to be biologically active. The most important conversion occurs via type 2 deiodinase (DIO2) in the brain, muscle, and brown fat.
Chronic inflammation suppresses DIO2 activity and upregulates DIO3, the enzyme that converts T4 to reverse T3 (rT3) — a biologically inactive form that occupies T3 receptors without activating them (Wajner & Maia 2012, Clinical Endocrinology). This is called “euthyroid sick syndrome” or “non-thyroidal illness syndrome” (NTIS). TSH is normal. T4 may be normal. But the T3 that actually drives metabolism in the tissues is low, and reverse T3 is high (Wajner and Maia 2012). The patient is functionally hypothyroid in every cell, and every lab the GP orders says everything is fine.
This is not a rare curiosity. It occurs in any chronic inflammatory state: sepsis, heart failure, chronic kidney disease, and — almost certainly — ME/CFS, where chronic immune activation is a central feature.
DIO2 genetic polymorphisms. The Thr92Ala polymorphism in the DIO2 gene (rs225014) is present in approximately 16% of the European population as homozygotes. It reduces DIO2 enzyme efficiency, impairing T4-to-T3 conversion in every tissue that relies on it. Carriers have normal TSH, normal T4, but may have functionally insufficient T3 at the cellular level (Panicker et al. 2009). This is not hypothyroidism by any standard definition. It is a genetically determined reduction in the efficiency of thyroid hormone activation that no standard thyroid panel detects.
For an otherwise healthy carrier, the deficit may be subclinical. For an ME/CFS patient — where every energy-producing system is already strained — a 10-15% reduction in T3 availability to mitochondria can be the difference between managing and not managing.
2 Hashimoto’s: the autoimmune destruction nobody checked for
Hashimoto’s thyroiditis is the most common autoimmune disease in the world. Antibodies (anti-TPO, anti-thyroglobulin) attack thyroid tissue, progressively destroying it. The destruction is slow — it takes years for enough tissue to be lost that TSH rises above the reference range.
During those years, the patient has active autoimmune thyroid inflammation, fluctuating hormone levels as intact follicles compensate for destroyed ones, and fatigue that correlates with antibody levels rather than with TSH (Ott et al. 2011). The TSH may stay in range for a decade while the gland is being destroyed. The patient has fatigue, brain fog, weight changes, cold intolerance, and hair loss — and is told their thyroid is fine because the only test ordered was TSH.
Anti-TPO antibodies are present in approximately 10% of the general population. In ME/CFS cohorts, the prevalence may be higher — the autoimmune diathesis that drives ME/CFS overlaps with the autoimmune diathesis that drives Hashimoto’s. Testing for antibodies adds approximately €20 to the lab order. It identifies a condition that is progressive, treatable, and completely invisible to TSH screening.
3 What a complete thyroid evaluation looks like
The minimum panel for a patient with chronic fatigue should include:
- TSH — necessary but not sufficient
- Free T4 — the prohormone. Low-normal + normal TSH suggests central hypothyroidism.
- Free T3 — the active hormone. Low free T3 with normal T4 and TSH = conversion problem.
- Reverse T3 — the inactive competitor. High rT3 with normal TSH = euthyroid sick syndrome.
- Anti-TPO and anti-thyroglobulin antibodies — Hashimoto’s screening. Positive = autoimmune thyroid destruction in progress, regardless of current TSH.
The complete panel costs approximately €50-80 at most European laboratories. The standard panel ordered by most GPs — TSH alone — costs €15 and misses every scenario described above.
4 The treatment controversy
Here is where the terrain becomes politically charged. The standard endocrinology position is: if TSH is normal, the patient is euthyroid, no treatment is indicated. This position is defensible by the evidence base if you define “euthyroid” by lab values and define “treatment response” by TSH normalisation.
It becomes less defensible when you ask whether patients with normal TSH but symptoms of hypothyroidism, low free T3, or positive antibodies improve with thyroid hormone replacement. Several studies suggest they do, at least in subgroups — particularly with combination T4/T3 therapy rather than T4 monotherapy, and particularly in carriers of the DIO2 Thr92Ala polymorphism ((Panicker et al. 2009); (Wiersinga et al. 2017)).
This is not an endorsement of prescribing thyroid hormone to everyone with fatigue and a normal TSH. It is an observation that the binary classification — euthyroid or hypothyroid, based on a single hormone from a single gland measured at a single time point — is too coarse to capture the reality of thyroid hormone action at the tissue level.
For ME/CFS patients specifically: the question is not whether sub-clinical thyroid dysfunction causes ME/CFS. It almost certainly doesn’t. The question is whether it contributes to the energy deficit in a subset of patients — and whether correcting it, in that subset, produces a meaningful improvement in function. The evidence is suggestive. The harm of a supervised trial of thyroid replacement in a patient with documented low free T3 and positive antibodies is minimal. The potential benefit, in a patient whose energy margin is measured in single-digit percentages, is significant (Loth 2026).
5 The deeper pattern
Thyroid dysfunction in ME/CFS is not a standalone problem. It is part of a system-wide pattern of endocrine suppression driven by chronic illness:
- HPA axis blunting (flat cortisol) → reduced ACTH → reduced cortisol → impaired stress response
- HPT axis blunting (central hypothyroidism) → reduced TSH → reduced thyroid output
- HPG axis suppression → reduced sex hormones → fatigue, body composition changes
- GH axis blunting → reduced growth hormone → impaired tissue repair and exercise recovery
These axes cross-talk. HPA suppression drags down the HPT axis. Chronic inflammation suppresses all of them. The result is a patient with “normal” values on every isolated endocrine test who is nonetheless functioning in a state of multi-axis endocrine insufficiency — not deficiency severe enough to trigger any individual alarm, but collectively sufficient to produce profound functional impairment.
This is the thyroid chapter of a larger story: the gap between “not flagged as abnormal” and “adequate for what the body needs” is the space where ME/CFS patients live. The thyroid is one axis. The pattern is the same across all of them.