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The 60-second version
If you read nothing else
- What it does — the thyroid makes two hormones, T4 and T3, that set the pace of nearly every cell: energy, body temperature, heart rate, mood, metabolism, and — critically — a baby's brain development.
- How it's built — hormones are assembled from one amino acid (tyrosine) and iodine, using a team of helpers: iron for the TPO enzyme, selenium to switch T4 into active T3, and zinc so T3 can act. Miss one and the line slows down.
- Why iodine is the headline — your body can't make it; you must eat it. Much of Europe — including the UK and Russia — is now mildly iodine-deficient again, and it hits women and babies first.1,2,3
- In pregnancy it's non-negotiable — the baby runs entirely on the mother's thyroid hormone until about week 20, so needs jump to 250 µg/day; low iodine early in pregnancy is linked to lower child IQ.4,5
- Read labs by optimal, not just 'normal' — a TSH of 3.5 or a bottom-of-range free T3 is "normal" on the report but often where symptoms live. Functional targets are tighter.
- But more is not always better — with iodine especially, the goal is enough, not mega-doses; excess can provoke autoimmune thyroiditis, particularly if selenium is low.6
The machinery
How your thyroid makes its hormones
The thyroid is a little assembly line. Raw materials arrive from the blood; the gland builds a hormone; helpers switch it on out in the tissues. Tap any step to see what it does and what it needs.
ↆ Click a step in the production line
The raw materials
The five building blocks
Every step above runs on a nutrient. Miss one and hormone production quietly slows — even when everything else is in place. Expand each to see what it does, where to find it, and what low looks like.
The headline nutrient
Iodine: why it's critical — and why pregnancy is different
Iodine has almost one job in the body: to build thyroid hormone. No iodine, no hormone. Your body can't make it and stores only a small amount, so you depend on a steady supply from food.
It's coming back as a problem. After decades of progress from iodised salt, mild iodine deficiency is rising again across Europe and North America, hitting women of reproductive age and pregnant women first. In the 2025 global scorecard, 23 countries are still iodine-deficient, and — mapped separately for the first time — pregnant women were inadequate in 15 of 24 European countries with data.1,3
The UK has no salt-iodisation programme, and is classed as mildly iodine-deficient; national surveys have found pregnant women and teenage girls below target, and children of mothers low in iodine in early pregnancy scored lower on IQ and reading.2,5
Russia is the only EAEU country without universal salt iodisation (Kazakhstan, Belarus, Kyrgyzstan and Armenia all have it), and iodine deficiency is a re-emerging problem affecting mothers and infants across a nation of 140+ million.7
Is mine off?
Signs & symptoms
Because the thyroid sets your metabolic pace, when it drifts the effects are wide-ranging — and easily blamed on stress, ageing or "just being tired". Broadly, a thyroid runs either too slow or too fast. These lists overlap with plenty of other conditions, so treat them as a prompt to test, not a diagnosis.
🐢 Underactive — everything slows down
- Persistent fatigue and heaviness, even after sleep
- Feeling cold; cold hands and feet
- Weight gain or trouble losing weight
- Constipation and sluggish digestion
- Dry skin, brittle or thinning hair (sometimes the outer eyebrow)
- Low mood, brain fog, poor concentration and memory
- Heavy or irregular periods; difficulty conceiving
- Puffiness, a hoarse voice, a slower heartbeat, muscle aches
⚡ Overactive — everything speeds up
- Anxiety, irritability, feeling wired or restless
- A racing or pounding heart, palpitations
- Weight loss despite a good (or bigger) appetite
- Feeling hot, flushed, sweating easily
- A fine tremor in the hands
- Insomnia; trouble winding down
- Frequent, looser stools; lighter or irregular periods
- Muscle weakness — and, in Graves', bulging or gritty eyes
Interpretation
Your labs, by optimal values
Most labs flag a result only when it falls outside a very wide "normal" range — built from a general population, many of whom are unwell. Functional/integrative practice reads the same numbers by a tighter optimal window, where most people actually feel well. Ranges vary by lab and by pregnancy — treat these as conversation-starters, not verdicts.
The brain's signal, TSH, is the number most panels report. Drag the slider (or type a value) to see where it lands — and remember a high TSH usually means the gland is being pushed to work harder.
The fuller panel, by optimal
Many conventional panels test only TSH. A fuller picture adds free T4, free T3, reverse T3 and antibodies. For each marker: what it is, the conventional "normal", and a tighter functional-optimal window.
| Marker | Conventional "normal" | Functional-optimal | How to read it |
|---|---|---|---|
| TSH the brain's signal |
≈ 0.4–4.0 mIU/L | ≈ 1.0–2.0 mIU/L | Higher-in-range often means the gland is being pushed. Lower in pregnancy (~<2.5 early). |
| Free T4 storage form |
≈ 12–22 pmol/L (0.9–1.7 ng/dL) |
upper third, ~15–19 pmol/L | The "raw" hormone waiting to be activated. Bottom-of-range is common in sluggish thyroids. |
| Free T3 the active hormone |
≈ 3.1–6.8 pmol/L (2.3–4.2 pg/mL) |
upper half/third | A bottom-of-range free T3 is "normal" on paper but is where many hypothyroid symptoms live. |
| Reverse T3 the "brake" |
lab-specific | low; healthy free-T3-to-rT3 ratio | Made from T4 under stress, illness, low iron or selenium; high rT3 flags poor conversion. |
| TPO & Tg antibodies the immune flag |
negative / low | negative / low | Raised antibodies point to autoimmune thyroid disease (Hashimoto's or Graves') — often years before TSH shifts. |
Reference ranges vary by lab, age, sex and pregnancy; a single value is a conversation-starter, not a verdict. Interpret it with your history and symptoms.
The named diseases
The common conditions, plainly
Most thyroid trouble comes down to a handful of conditions. Here they are in plain terms — what they are, and which way they push the gland.
🛡️ Hashimoto's thyroiditis underactive
The most common cause of an underactive thyroid where iodine is sufficient. The immune system slowly attacks the gland (you see raised TPO / Tg antibodies), and over years hormone output declines. It's often silent early — TSH just creeps up. Selenium may lower antibodies; very high iodine can aggravate it. Treated with levothyroxine (T4) when the gland can no longer keep up.
🔥 Graves' disease overactive
The most common cause of an overactive thyroid. A different antibody (TRAb) pushes the gland to overproduce, driving the "too fast" symptoms and sometimes bulging eyes. It needs medical treatment — anti-thyroid medication, radioiodine, or surgery — and shouldn't be self-managed.
🫧 Nodules & goitre
A goitre is a swollen gland (classically from iodine deficiency, as it strains to catch more); nodules are lumps within it. Most are benign and many need only monitoring — but a new or growing lump should always be checked by a clinician.
🔄 Postpartum & subacute thyroiditis
Temporary inflammation — after pregnancy, or following a viral illness — that can swing the thyroid from over- to under-active over weeks or months. It often settles on its own, but is worth recognising rather than mistaking for anxiety then depression.
The obstacles
What gets in the way
Even with the raw materials in place, a few everyday things can hold the thyroid back — or distort the test itself. Worth knowing before you blame the gland.
😮💨 Stress & cortisol
Chronic stress shifts conversion away from active T3 and toward the inactive reverse T3 "brake", and can quiet the brain's signal. It won't usually cause disease on its own, but it can make a marginal thyroid feel worse.
🦠 Gut health
A meaningful share of T4→T3 conversion, and the absorption of iron, selenium and zinc, depends on a healthy gut. Coeliac disease is more common alongside Hashimoto's — worth bearing in mind if things don't add up.
🥦 Goitrogens, in perspective
Very large amounts of raw cruciferous veg (cabbage, kale) or soy can mildly interfere with iodine use — but mostly only when iodine is already low. Normal, cooked portions are fine and healthy. Don't give up your broccoli; just keep iodine covered.
🧴 Competing halides functional view
Fluoride and bromide sit near iodine chemically, and some functional practitioners worry they compete with it. Honest read: human evidence is weak. Getting enough iodine matters far more than fearing everyday exposure.
Not a single point of view
Two ways to read it
The same numbers get read two ways. Both are defensible — they're answering slightly different questions. Here they are, side by side.
🩺 How conventional medicine plays it
- Relies mainly on TSH (± free T4) to diagnose an over- or under-active thyroid.
- Treats when values fall outside the wide reference range; standard treatment for low thyroid is levothyroxine (T4).
- Doesn't usually chase symptoms while labs read "in range".
- Cautious about routine T3, reverse T3 or micronutrient testing.
- Strength: rigorous and trial-anchored. Blind spot: can dismiss symptoms while the number is technically "normal".
🌿 How functional / integrative medicine plays it
- Reads a fuller panel — TSH, free T4, free T3, reverse T3, TPO/Tg antibodies — against tighter optimal ranges.
- Looks upstream at the raw materials (iodine, tyrosine, iron, selenium, zinc), the gut and stress.
- Treats the person and their symptoms, not just the number.
- Fair critique: optimal ranges are partly consensus, not hard outcome-trials; extra tests cost more; and "optimising" a marker isn't the same as a proven outcome — but the inputs it targets are cheap and low-risk.
🩺 Conventional, in a line
- Question it answers: "Is this outside the range, and does treating it change hard outcomes?"
- Thyroid labs ≈ a diagnostic gate: in-range means no disease.
- Best when a clear diagnosis and a proven drug are the goal.
🌿 Functional, in a line
- Question it answers: "Is this system strained, and can I support it safely?"
- Thyroid labs ≈ a dashboard to optimise, alongside raw materials and symptoms.
- Best when someone feels unwell but sits "in range".
A softer lens
A gentle note
Many holistic traditions link the throat and thyroid with voice and self-expression — with holding back, over-adapting, or swallowing what we really want to say. There's no lab test for this, and it's not a diagnosis. But it can be a kind invitation: alongside the nutrients and the numbers, notice whether you feel free to speak up and take up space. Body and mind aren't separate.
What to do
What this means in practice
A tidy checklist
- Get a fuller panel if you have symptoms — not TSH alone. Add free T4, free T3, and TPO/Tg antibodies; check ferritin too.
- Cover the raw materials with food: iodine (seafood, dairy, eggs, iodised salt), protein for tyrosine, iron, selenium (1–2 Brazil nuts), and zinc.
- Pregnant or planning? Ensure a prenatal with ~150 µg iodine, and don't rely on non-iodised sea or pink salt. Aim for the pregnancy target with your clinician.
- Iodine: aim for enough, not mega-doses. Pair it with selenium, and individualise if you have autoimmune thyroid disease.
- Read results by optimal + symptoms, and interpret them with a clinician who knows your history.
Keep reading — it's free
You've got the part that shows you where you stand. The rest is what to actually do about it — and I'll send it straight to your screen.
Still to come in this guide:
- The common conditions, plainly
- What gets in the way
- Two ways to read it
- A gentle note
- What this means in practice
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Go deeper
Sources
The evidence behind the numbers above. Superscript numbers in the text map to this list.
- Iodine Global Network — Global Scorecard of Iodine Nutrition 2025. Country-by-country iodine status; 23 deficient countries and pregnant-women mapping.
- British Dietetic Association — Iodine deficiency in the UK: dietetic implications. The UK's lack of a salt-iodisation programme and at-risk groups.
- Iodine status of the population in the WHO European Region. Re-emerging deficiency across Europe, including in pregnancy.
- Iodine deficiency in pregnancy and maternal supplementation — review. The 250 µg/day target and fetal brain development.
- A review of the iodine status of UK pregnant women and implications for the offspring. Links to child IQ and reading scores.
- Selenium, Iodine and Iron — Essential Trace Elements for Thyroid Hormone Synthesis and Metabolism. Cofactors, conversion, and the iodine-excess/autoimmunity caution.
- Iodine Status of Women and Infants in Russia: A Systematic Review. Russia and universal salt iodisation.