Reasoning GP · Algorithm · 1-page summary
Abnormal thyroid function tests
For: adults in primary care with an abnormal TSH, free T4 or free T3. Not for: pregnancy (RCOG Green-top 76 and the local antenatal endocrine pathway), children and young people (specialist pathways), thyroid cancer follow-up (specialist team's plan). Ongoing care: hypothyroidism and hyperthyroidism protocols.
Core rule: read TSH and free T4 together as a pattern, confirm a subclinical result on repeat before treating, and never wait for TFTs when a patient looks like thyroid storm or myxoedema coma.
0Stop — call 999 now — both are clinical diagnoses
- Thyroid storm: suppressed TSH or known/suspected thyrotoxicosis + fever + marked tachycardia or AF + agitation, confusion or reduced consciousness, heart failure, or vomiting, diarrhoea or jaundice → 999 immediate transfer; discuss with critical care / endocrinology.
- Myxoedema coma: known or suspected hypothyroidism (often missed doses) + drowsiness, confusion or coma + hypothermia; bradycardia, hypoventilation → 999
- New AF + suppressed TSH, haemodynamically unstable (shock, syncope, chest pain, heart failure) → 999
- While waiting: ABCDE, ECG, glucose, temperature only if no delay. No score or drug regimen before transfer.
1Triage — TSH / free T4 pattern
| Pattern | Meaning → next step |
|---|---|
| TSH ↓ · FT4 ↑ | Hyperthyroidism → TRAb, refer endocrinology |
| TSH ↓ · FT4 normal | Subclinical hyperthyroidism (FT3: T3 toxicosis?) → repeat to confirm |
| TSH ↑ · FT4 ↓ | Overt primary hypothyroidism → levothyroxine |
| TSH ↑ · FT4 normal | Subclinical hypothyroidism → repeat in ~3 months; TPO once |
| Discordant | Low FT4, TSH not raised → possible central (or illness, drugs, assay). High FT4, TSH not suppressed → biotin, assay, recent dose, resistance, TSH-oma → lab + endocrinology |
2Screen — serious causes
- Thyroid eye disease with reduced acuity or colour vision, corneal exposure, severe pain or optic neuropathy → Same-day ophthalmology.
- Carbimazole + sore throat, fever, mouth ulcers, bruising or malaise → stop, FBC Same-day
- Unexplained thyroid lump, whatever the TSH → consider Suspected cancer referral (NG12).
- Suspected central hypothyroidism → 9 am cortisol before levothyroxine → Urgent endocrinology.
- New overt hyperthyroidism, amiodarone thyrotoxicosis, thyroid eye disease → Urgent endocrinology.
- Stable new AF + suppressed TSH → ECG, AF care, endocrinology ≤7 days
3Assess
☐ Symptoms, eyes ☐ Amiodarone, lithium, interferon, checkpoint inhibitors, contrast; biotin (false low TSH, high FT4) ☐ Pregnancy, postpartum, recent virus ☐ Gland, nodes, pulse rhythm, BP, weight ☐ Pituitary suspected → TSH + FT4 together
Not in primary care: isotope scan, FNA, ultrasound in thyrotoxicosis without a nodule.
4Classify — only if blocks 0–2 are negative
- Graves' — TRAb positive, diffuse goitre, eye disease → endocrinology.
- Toxic nodular goitre / adenoma — nodular, older, TRAb negative → endocrinology.
- Thyroiditis — painful after a virus, or painless postpartum; TRAb negative; no antithyroid drug.
- Hashimoto's / drug-induced — TPO antibodies; on amiodarone → levothyroxine if symptomatic or TSH >10.
- On levothyroxine, TSH 5–10 — undertreated, not subclinical → titrate up.
- Uncertain — repeat; laboratory or endocrinology Advice & Guidance.
5Treat today — only these
- Overt hypothyroidism: levothyroxine about 1.6 mcg/kg/day if <65 without cardiovascular disease; ≥65 or cardiovascular disease → consider 25–50 mcg, titrate cautiously. TSH every 3 months until stable, then yearly; aim in range, not suppressed.
- Subclinical hypothyroidism: TSH ≥10 on two tests about 3 months apart → consider treatment. Above range but <10 twice, symptoms, <65 → consider a 6-month trial; if no better, consider stopping.
- Hyperthyroidism: refer; consider a beta-blocker while waiting. Carbimazole only by, or on advice of, the specialist: baseline FBC + LFTs, agranulocytosis warning, effective contraception.
- Levothyroxine on an empty stomach; calcium, iron ≥4 h apart. Doses: SPC, local formulary.
6Refer routinely — and monitor
- Subclinical hyperthyroidism, TSH <0.1 on 2 readings ≥3 months apart + thyroid disease or symptoms → consider specialist advice.
- Untreated subclinical hypothyroidism: TSH + FT4 yearly if TPO-positive or previous surgery / radioiodine; otherwise every 2–3 years.
- Stable on levothyroxine: annual TFT; sooner for pregnancy, new medicines, weight change, malabsorption.
- Symptoms despite TSH in range: FT4 and other causes (coeliac, B12, anaemia, sleep, mood); T3 combination is a specialist decision.
7Never reassure by
- A suppressed TSH or "toxic nodule" when there is a thyroid lump.
- Waiting for TFTs or a score in a febrile, confused, tachycardic patient.
- Non-pregnant reference ranges in a pregnant woman.
- One borderline TSH, or TFTs taken during acute illness.
- A TSH alone when pituitary disease or biotin use is possible.
- Stopping amiodarone yourself — that is a cardiology decision.
8Safety-net — say it, then document it
999 “Call 999 if you become feverish, confused or agitated with a racing heartbeat, or very drowsy, cold and confused — especially if doses have been missed.” Same-day “On carbimazole: a sore throat, fever, mouth ulcers, bruising or feeling very unwell → stop it and get a blood test today; out of hours, NHS 111 or A&E.”
Pregnant on levothyroxine: increase the dose by 25–30% at once (e.g. two extra doses a week), TFTs within 4 weeks, antenatal endocrine pathway. Follow-up: next TFT date booked.
✓Safe to leave primary care only if — all five, documented
No block 0–2 trigger applies
TSH + FT4 pattern read; biotin, drugs, illness checked
Pregnancy status known
Repeat or next TFT date set; result owner named
Safety-net given and documented
Summary of the full Reasoning GP abnormal TFTs algorithm, which holds pregnancy, amiodarone, radioiodine, surgery and monitoring detail. Sources: NICE NG145, NG12 (updated April 2026), NG196; RCOG Green-top 76; BTA/BSPED 2019; EUGOGO 2021; MHRA (carbimazole, levothyroxine); local formulary and amiodarone shared care. Clinical reference — follow local pathways and check doses in the SPC and your local formulary.