How to read your thyroid panel — beyond TSH
TSH is a pituitary hormone, not a thyroid hormone. Using it as your sole marker is like judging engine health by accelerator position. Here's what a complete panel actually tells you.
A TSH-only thyroid screen is one of the most persistent misuses of biochemistry in routine medicine. The number tells you what the pituitary thinks about thyroid output — filtered through hypothalamic set-points, deiodinase activity inside the pituitary itself, and a feedback loop that lags real tissue status by weeks. If you already accept that TSH alone is insufficient, this piece is for you. We will treat the panel as a system: signal, substrate, active hormone, inactive competitor, and autoimmune trajectory.
TSH is a pituitary signal, not a thyroid measurement
TSH is secreted by the anterior pituitary in response to falling intra-pituitary T3. The critical word is intra-pituitary. Pituitary tissue runs its own type 2 deiodinase (D2) which converts T4 to T3 locally — and it does so with enzyme kinetics that differ from peripheral tissues. The pituitary is therefore the worst possible sentinel for what is happening in muscle, brain, gut, or liver, because it has privileged access to T3 that those tissues do not.
A TSH of 2.8 mIU/L is "normal" by reference range and "high-normal" by every meaningful clinical literature on optimal thyroid function. Population studies of healthy euthyroid adults without anti-thyroid antibodies cluster the median TSH around 1.0–1.4 mIU/L. When you see a TSH drifting upward year-on-year inside the reference range, you are watching a feedback loop work harder to extract the same hormonal output — the pituitary asking louder.
TSH rises only when the pituitary's own intracellular T3 falls. By the time TSH crosses the upper reference limit, peripheral tissues have usually been hypothyroid for months to years. The lag is biological, not technical.
A suppressed TSH (< 0.1 mIU/L) means the pituitary has decided the body has enough — or too much — circulating thyroid hormone. That decision can be wrong in two directions, which we cover in the final section.
The T4 to T3 conversion cascade
The thyroid gland secretes roughly 80 percent T4 and 20 percent T3 by molar output. T4 is a prohormone. Tissue activity depends almost entirely on local conversion of T4 to T3 by deiodinase enzymes:
- D1 (liver, kidney): supplies systemic T3, scavenges reverse T3
- D2 (brain, pituitary, brown fat, muscle): generates intracellular T3 for nuclear receptors
- D3 (placenta, brain, inflamed tissue): inactivates T4 to reverse T3 and T3 to T2
Conversion is not a passive chemistry experiment. It requires selenium as a cofactor (selenocysteine sits in the active site of all three deiodinases), adequate iron, zinc, and intact mitochondrial redox status. Deiodinase activity is suppressed by elevated cortisol, systemic inflammation (IL-6, TNF-alpha), prolonged caloric restriction, low leptin, and reverse T3 itself in a self-reinforcing loop.
Cortisol upregulates D3 and downregulates D1. Low-calorie dieting downshifts D2 within 72 hours. Inflammation drives D3 in peripheral tissue. Selenium below 100 mcg/L plasma silently caps conversion no matter how much T4 you have.
If you suspect conversion failure, a cortisol and DHEA panel run alongside the thyroid panel will often expose the upstream driver. Stress-pattern cortisol with suppressed DHEA pairs almost universally with low Free T3 and elevated reverse T3.
Free T3: the active hormone
Free T3 binds nuclear receptors. It sets basal metabolic rate, mitochondrial biogenesis, cardiac output sensitivity to catecholamines, and the rate of protein turnover. This is the molecule that does the work.
The reference range typically spans 2.0–4.4 pmol/L. Symptomatic hypothyroidism — cold intolerance, slow Achilles reflex, constipation, dry skin, brain fog, mid-afternoon collapse — appears reliably below 3.0 pmol/L even when TSH is unremarkable. A Free T3 sitting at 2.6 pmol/L with a TSH of 1.9 mIU/L is a classic missed diagnosis: lab software flags neither value, but the patient is hypothyroid at the tissue level.
The optimal window for high-functioning adults is the upper third of range, 3.5–4.2 pmol/L. Above 4.4 pmol/L without exogenous T3 is rare and worth investigating. Above 4.4 pmol/L with suppressed TSH and hyperadrenergic symptoms is thyrotoxicosis until proven otherwise.
Free T4: the reservoir
Free T4 represents the unbound, biologically available thyroxine pool — the substrate from which active hormone is generated.
The interpretive value of Free T4 lies almost entirely in its relationship to Free T3. Three patterns matter:
- Free T4 mid-range, Free T3 upper third: healthy conversion, intact deiodinase activity
- Free T4 upper range, Free T3 lower third: conversion failure — the tank is full, the engine is starved
- Free T4 lower third, Free T3 lower third: primary hypothyroidism or central (pituitary) hypothyroidism if TSH is also low
Low Free T4 with normal Free T3 is the early signature of a thyroid gland under strain compensating by upregulating peripheral conversion. It is often the first measurable shift in autoimmune thyroiditis before TPO antibodies become detectable.
Reverse T3: the inactive competitor
Reverse T3 (RT3) is the mirror-image isomer of T3. It binds the same nuclear receptors with comparable affinity but produces no transcriptional activity. It is a clean antagonist.
When D3 activity rises — driven by cortisol, inflammation, severe illness, starvation, surgery, sepsis, or chronic over-training — T4 is shunted away from active T3 and into RT3. Circulating RT3 then competes with whatever Free T3 is still being produced for receptor occupancy. The biological effect is a deeper functional hypothyroidism than the Free T3 number alone suggests.
Aggressive caloric deficits below 1,400 kcal sustained beyond two weeks. Untreated sleep apnoea. Chronic infection or autoimmune flare. Heavy metal load. High-volume endurance training without adequate recovery. Beta-blockers (which themselves suppress conversion).
A reverse T3 above 20 ng/dL in the context of a Free T3 in the lower half of range is the hallmark of "low T3 syndrome" — sometimes labelled euthyroid sick syndrome — and it is rarely euthyroid in any meaningful sense. The fix is upstream: address the cortisol, the inflammation, the deficit, the infection. Adding more T4 makes RT3 worse. Sex hormone status compounds the picture; review the testosterone panel interaction below before reaching for thyroid replacement.
TPO antibodies: the autoimmune piece
Thyroid peroxidase antibodies (TPO Ab) target the enzyme that catalyses iodination of thyroglobulin. They are the single most predictive marker of future thyroid failure — even when TSH, Free T3, and Free T4 are pristine today.
Persistent TPO Ab above 100 IU/mL carries roughly a 2–4 percent annual risk of progression to overt hypothyroidism. Across a decade, that compounds meaningfully. Antibodies above 500 IU/mL frequently coincide with a fluctuating TSH that swings through reference range without ever sitting still — a pattern produced by intermittent immune-mediated cytotoxic release of stored hormone followed by recovery.
A positive TPO antibody result with a normal panel is not a diagnosis to ignore. It is a forecast. Selenium 200 mcg daily has reproducible data on antibody titre reduction. Gluten removal has weaker but suggestive data, particularly in those with concurrent coeliac genetics. Low-dose naltrexone is used clinically and has small trial data. Vitamin D status is also relevant here — studies consistently find lower 25-OH vitamin D in Hashimoto's and Graves' patients compared to controls, and some intervention trials show supplementation reduces TPO antibody titres; see the vitamin D optimal levels guide for what serum levels to target.
Reading the panel as a system
Single values are cheap. Ratios are diagnostic.
Free T3 to Free T4 ratio (using consistent units): a healthy ratio sits around 0.30 when both are expressed in pmol/L. Below 0.24 suggests conversion failure regardless of where individual values fall. Above 0.40 suggests either exogenous T3 dosing or aggressive D2 upregulation in response to peripheral T3 demand.
Reverse T3 to Free T3 ratio: the most under-used metric in thyroid interpretation. With RT3 in ng/dL and Free T3 in pg/mL, a ratio under 10 is favourable, 10–20 is suspect, and above 20 indicates significant deiodinase displacement. If your lab reports Free T3 in pmol/L, divide by 1.536 to convert to pg/mL before calculating.
Optimal panel: TSH 1.0–2.0, Free T4 mid-upper range, Free T3 upper third, RT3 below 15 ng/dL, TPO Ab below 35. Anything that breaks this constellation is worth a second look — even if every individual value is "in range".
Suppressed TSH in context
A TSH below 0.1 mIU/L means the pituitary has shut off its signal. Three common interpretations:
- Primary hyperthyroidism: Free T3 and Free T4 above range. Usually Graves' or toxic nodular disease. Investigate.
- Exogenous T3 or T4 replacement: TSH suppression is expected and not pathological if Free T3 is in the optimal window without symptoms of thyrotoxicosis. Many endocrinologists target a TSH of 0.3–1.0 on replacement; aiming for a "normal" TSH on T4 monotherapy often leaves Free T3 chronically low.
- Central hypothyroidism: TSH inappropriately low or low-normal alongside low Free T4 and low Free T3. Pituitary or hypothalamic origin. Requires imaging and a wider pituitary workup.
The clinical picture, not the TSH alone, decides which scenario applies. A suppressed TSH in an asymptomatic person on physiologic T3 with optimal Free T3 is not a problem to fix. A suppressed TSH with elevated Free T3 and tachycardia, weight loss, and tremor is.
The thyroid panel rewards readers who treat it as a network of interacting signals rather than a checklist. Run the full panel. Calculate the ratios. Track antibodies even when the rest looks clean. The cost of a complete picture is one extra blood draw; the cost of TSH-only screening is years of suboptimal function dressed up as normal. For how the thyroid panel fits within a broader biomarker review — including CBC, metabolic, liver, kidney, and hormone markers — see the complete guide to interpreting blood test results.
If you are in Australia and want to access a full thyroid panel — including Free T3, Free T4, reverse T3, and TPO antibodies — without waiting for a GP referral, see our guide to private blood testing in Australia for the self-referral providers that carry these markers and the current pricing you can expect.