Thyrotoxicosis: causes and initial assessment

01OverviewDefinition, clinical context and the essential points that orientate the chapter.

The first task is severity, not cause. A patient with mild tremor and a pulse of 96 is managed differently from a febrile, delirious person with heart failure, even if their free T4 values are identical. Thyroid storm is a clinical syndrome of decompensated organ dysfunction rather than a laboratory threshold. Older adults may instead present with weight loss, atrial fibrillation, heart failure or apathy and few adrenergic symptoms.

Once stable, mechanism directs treatment. In Graves and autonomous nodules, the gland actively synthesises hormone and antithyroid medicine can inhibit production. In thyroiditis, preformed hormone leaks from damaged follicles, so thionamides add adverse effects without addressing the mechanism. Exogenous hormone, iodine exposure and amiodarone introduce further branches that require careful history and specialist interpretation.

Cause is built from examination, TRAb and selective imaging. A diffuse vascular goitre plus eye disease strongly supports Graves; an irregular nodular gland in an older adult suggests autonomy; a tender gland with raised inflammatory markers suggests subacute thyroiditis. Scintigraphy distinguishes diffuse, focal, patchy and low uptake, but is unsuitable during pregnancy and requires nuclear-medicine precautions in breastfeeding.

Key points

Thyrotoxicosis means tissue exposure to excess thyroid hormone; hyperthyroidism specifically means increased gland synthesis, so the terms are not interchangeable.
Typical features include weight loss despite appetite, heat intolerance, sweating, tremor, anxiety, proximal weakness, frequent stools, menstrual change and tachycardia.
Suppressed TSH with raised free T4 and/or free T3 confirms overt biochemical thyrotoxicosis after assay and medicine confounders are considered.
Graves disease, toxic multinodular goitre and toxic adenoma produce hormone synthesis; subacute, painless or postpartum thyroiditis releases stored hormone; excess levothyroxine is exogenous.
Examine pulse and rhythm, blood pressure, temperature, hydration, mental state, heart failure, muscle power, goitre or nodules, thyroid bruit and eye signs at the first encounter.
NICE advises TSH-receptor antibodies to confirm Graves; if negative and the cause remains uncertain, consider technetium scintigraphy through the specialist pathway.
Ultrasound is not a routine test for biochemical thyrotoxicosis and is mainly indicated when examination finds a palpable nodule or another structural concern.
Before starting an antithyroid medicine, obtain full blood count and liver tests, discuss agranulocytosis symptoms, pregnancy and contraception, and check the current BNF.
Antithyroid medicines do not treat destructive thyroiditis because new synthesis is already suppressed; symptom control and time are usually the core approach.
Pregnancy or planned conception requires prompt specialist care because investigations, drug selection, dose targets and radioiodine eligibility change.

02AetiologyUnderlying causes, associations and risk factors, with why each one matters.

Increased thyroid synthesis

Graves disease and autonomous nodules actively produce excess hormone, creating a mechanism that can respond to synthesis-blocking treatment.

Destructive hormone release

Painful, painless or postpartum thyroiditis releases stored hormone from damaged follicles without increasing new synthesis, usually creating a self-limited thyrotoxic phase.

Exogenous or iodine-related exposure

Prescribed thyroid hormone, supplements, iodine and amiodarone can produce or complicate thyrotoxicosis through distinct mechanisms when supported by the surrounding clinical evidence.

03PathophysiologyThe causal sequence from the underlying abnormality to symptoms and harm.

1
Circulating thyroid hormone rises
Increased gland synthesis, follicular leakage or external exposure raises T4, T3 or both and suppresses pituitary TSH.
2
Metabolic demand increases
Tissues generate more heat and consume more energy, producing weight loss, sweating, muscle catabolism and increased gut activity.
3
Cardiovascular sensitivity increases
Adrenergic responsiveness and cardiac workload rise, causing tachycardia, atrial fibrillation, widened pulse pressure and sometimes heart failure.
4
Organ reserve determines severity
Older age, cardiac disease and an acute trigger can produce major decompensation despite less striking tremor or laboratory differences.

04Clinical features and red flagsSymptoms, examination findings, patterns of presentation and time-critical warnings.

Adrenergic thyrotoxicosis

Fine tremor, warm moist skin, brisk reflexes, lid lag, anxiety, sinus tachycardia and proximal myopathy accompany weight loss and heat intolerance, though severity and presentation vary with age and beta-blocker use.

Cardiovascular presentationRed flag

Atrial fibrillation, systolic hypertension, high-output failure, worsening angina or unexplained dyspnoea may dominate, especially in older people; establish haemodynamic stability before outpatient planning.

Graves phenotype

A smooth diffuse goitre, thyroid bruit, lid retraction, proptosis, restrictive diplopia, pretibial dermopathy or acropachy suggests autoimmune Graves, but TRAb is used for biochemical confirmation.

Nodular autonomy

An irregular multinodular gland or solitary nodule, gradual onset and absence of extrathyroidal Graves signs suggest toxic multinodular goitre or adenoma, often with disproportionate T3 elevation.

Destructive inflammation

Painful tenderness after a viral-like illness supports subacute thyroiditis, while painless and postpartum forms lack tenderness; each can pass from a thyrotoxic phase through temporary hypothyroidism.

Decompensated organ dysfunctionRed flag

High fever, delirium, vomiting or diarrhoea, jaundice, marked tachyarrhythmia and cardiac failure indicate possible thyroid storm. Begin critical-care treatment on clinical suspicion without waiting for TRAb or imaging.

05InvestigationsWhat to request, why it matters and how to interpret it.

Investigation order

Read from the initial assessment onwards. Tests may run in parallel in urgent care; first-line, preferred, confirmatory, definitive and gold-standard labels appear only when the chapter explicitly states them.

01
TSH, free T4 and free T3First step
Why
Confirm overt thyrotoxicosis and identify T3-predominant disease.
Interpretation and limitations
Suppressed TSH plus either peripheral hormone above range confirms the biochemical state. Normal free T4 alone does not exclude T3 toxicosis; concentrations do not grade storm.
02
ECG and cardiovascular observations
Why
Identify arrhythmia, ischaemia and haemodynamic consequences immediately.
Interpretation and limitations
Sinus tachycardia and atrial fibrillation are common; heart failure, hypotension, dynamic ischaemia or broad-complex rhythm moves management into an emergency pathway.
03
TSH-receptor antibodies
Why
Confirm Graves disease after biochemical thyrotoxicosis is established.
Interpretation and limitations
A positive TRAb supports Graves and can avoid diagnostic imaging. A negative result prompts reconsideration and possible specialist scintigraphy rather than excluding Graves absolutely.
04
Full blood count and liver function tests
Why
Establish baseline safety before antithyroid medication and assess systemic illness.
Interpretation and limitations
Pre-existing neutropenia or significant hepatic abnormality needs specialist discussion. Baseline values help interpret later fever, sore throat, jaundice or abdominal symptoms.
05
Inflammatory markers
Why
Support subacute painful thyroiditis when the clinical picture fits.
Interpretation and limitations
Marked ESR or CRP elevation with a tender gland supports de Quervain thyroiditis but infection and other inflammatory causes must still be considered.
06
Technetium thyroid scintigraphy
Why
Differentiate diffuse Graves uptake, autonomous nodules and low-uptake thyroiditis.
Interpretation and limitations
Use when TRAb is negative or mechanism remains uncertain; diffuse uptake suggests Graves, focal or patchy uptake autonomy, and low uptake destructive or exogenous causes.
07
Pregnancy test when relevant
Why
Prevent inappropriate imaging, radioiodine or medicine exposure.
Interpretation and limitations
A positive result requires prompt joint endocrine-obstetric management; gestational transient thyrotoxicosis and Graves need careful clinical and antibody distinction.

06Differential diagnosisRealistic alternatives and the features that help distinguish them.

Anxiety or stimulant exposure

Tremor, sweating and palpitations overlap, but suppressed TSH with raised free hormones supports genuine thyrotoxicosis when biochemical results match the presentation.

Sepsis

Fever, tachycardia and weight loss can reflect infection, which also distorts thyroid tests and can precipitate thyroid storm.

Phaeochromocytoma

Episodic headache, pallor and labile hypertension with metanephrine elevation suggest catecholamine rather than thyroid hormone excess.

Non-thyroidal illness

Acute systemic disease can suppress TSH transiently without a coherent raised-hormone pattern or thyroid phenotype when biochemical results match the presentation.

07ManagementImmediate care, first-line treatment, alternatives and escalation.

01TriageGrade clinical severity firstFirst stepA new thyroid profile or presentation suggests thyrotoxicosis.

1Perform ABCDE and document temperature, pulse rhythm, blood pressure, oxygenation, hydration, cognition, heart failure, chest pain and gastrointestinal or hepatic features.
2EscalationEscalate fever plus organ dysfunction, unstable arrhythmia, pulmonary oedema or delirium to the thyroid-storm and critical-care pathway immediately.
3If physiologically stable, obtain ECG and key bloods, assess pregnancy and arrange prompt endocrine or local pathway follow-up matched to severity.

02CauseIdentify synthesis versus releaseOvert biochemical thyrotoxicosis is confirmed without immediate decompensation.

1Review goitre pattern, eye disease, thyroid pain, postpartum timing, prescribed or non-prescribed hormone, amiodarone, iodine contrast and family or autoimmune history.
2Request TRAb for suspected Graves; add inflammatory markers for a painful gland and avoid reflex ultrasound without a structural examination finding.
3When antibody and clinical evidence remain inconclusive, involve endocrinology for scintigraphy, respecting pregnancy and breastfeeding restrictions.

03Initial symptom controlReduce adrenergic and cardiac burdenTremor, palpitations or tachycardia is troublesome while cause-specific care is arranged.

1AlternativeAssess asthma, heart block, blood pressure, decompensated heart failure and pregnancy before choosing a beta blocker or alternative strategy.
2Use a BNF-checked short-term regimen with a clinical target, while separately evaluating and treating atrial fibrillation, heart failure and thromboembolic risk.
3DefinitiveReview frequently and reduce symptom medicine as euthyroidism returns; beta blockade does not correct hormone production or replace definitive treatment.

04Antithyroid startPrescribe with safety-nettingIncreased thyroid synthesis is likely and specialist or local guidance supports thionamide treatment.

1Check full blood count, liver tests, pregnancy status, contraception needs, medicine interactions and the cause-specific plan before the first dose.
2Explain to stop the drug and seek urgent full blood count for fever, sore throat, mouth ulcers or infection; give specific advice for jaundice or pancreatitis symptoms.
3EscalationArrange thyroid-function monitoring and endocrine review, noting that TSH can remain suppressed after free hormones improve and should not drive early escalation.

Key medicines and prescribing safety2 treatments · regimens, roles and cautions

Controls tremor, palpitations and tachycardia during the early thyrotoxic phase.

Propranolol for adrenergic symptoms

A common adult BNF range is 10 to 40 mg by mouth three or four times daily, adjusted to pulse, symptoms and comorbidity while the underlying cause is treated.

Avoid or modify with asthma, significant bradycardia, heart block, hypotension or decompensated low-output failure. Pregnancy, older age and interacting rate-control therapy need individual review.

Inhibits new hormone synthesis in Graves disease and autonomous nodular hyperthyroidism.

Carbimazole for hormone synthesis

Initial adult dosing is severity-dependent and commonly totals 15 to 40 mg daily in the BNF; use the specialist plan, then reduce or block-and-replace only under the agreed protocol.

Obtain baseline blood count and liver tests. Stop and seek urgent assessment for infection symptoms; discuss teratogenic risk, contraception, pregnancy, jaundice and pancreatitis, and follow current MHRA advice.

08ComplicationsImportant consequences, why they occur and why they matter clinically.

Atrial fibrillation and embolism

Persistent thyroid-hormone excess promotes atrial fibrillation, with associated risks of embolic stroke and heart failure, particularly in older adults.

Heart failure

Sustained tachycardia, high-output stress and underlying cardiac disease can produce ventricular dysfunction, pulmonary oedema and haemodynamic compromise.

Bone loss

Persistent thyroid-hormone excess accelerates bone turnover, progressively reducing bone density and increasing later fragility-fracture and osteoporosis risk.

Thyroid storm

Infection, surgery or another acute trigger can convert compensated thyrotoxicosis into hyperthermic cardiovascular, neurological and gastrointestinal organ failure.

09Monitoring and follow-upTreatment response, safety checks and longer-term review.

Recheck free T4 and free T3 at the local guideline interval after starting treatment; early TSH may stay suppressed and can mislead dose adjustment.
Review pulse, rhythm, weight, tremor, temperature, blood pressure and heart-failure symptoms alongside biochemistry rather than treating a laboratory value alone.
Do not routinely repeat full blood count or liver tests solely because carbimazole continues; investigate promptly when infection, mouth ulcers, jaundice, pruritus or abdominal symptoms occur.
Ask about pregnancy intentions and perform pregnancy assessment whenever clinically relevant before scintigraphy, radioiodine or antithyroid treatment decisions.
Track cause-specific milestones: TRAb status and eye symptoms in Graves, nodule imaging plans in autonomy, and hyper-to-hypothyroid phase transition in thyroiditis.
Reassess adherence and medicine exposure when free hormones fail to fall, including iodine, amiodarone and prescribed or non-prescribed thyroid hormone.

10Special situationsVariants, exceptions and circumstances that change the usual approach.

Mechanism determines usefulness

A thionamide blocks synthesis, so it helps Graves or autonomy but cannot retrieve hormone already released by a destructively inflamed gland.

Older adults can look apathetic

Weight loss, muscle weakness, atrial fibrillation or heart failure may occur with little tremor or agitation, making pulse and biochemistry important when presentation is muted.

T3 completes low-TSH testing

Early Graves and toxic nodules may raise free T3 before free T4, so a normal free T4 must not close the diagnostic pathway.

Ultrasound cannot show turnover

Structural appearance does not reliably separate hormone synthesis from follicular leakage; antibodies and selective functional imaging answer that physiological question better.

Storm is an organ diagnosis

No free-hormone cutoff distinguishes storm from uncomplicated disease. Fever, brain dysfunction, cardiovascular collapse and gastrointestinal or hepatic involvement create the emergency.

11Common pitfallsFrequent interpretation and management errors.

01
Using thyrotoxicosis and hyperthyroidism as exact synonyms and giving thionamide for thyroiditis.
02
Waiting for TRAb or scintigraphy while a delirious febrile patient deteriorates.
03
Omitting free T3 when TSH is suppressed and free T4 remains normal.
04
Ordering ultrasound to determine the cause despite no palpable structural abnormality.
05
Starting carbimazole without baseline blood count, liver tests and pregnancy counselling.
06
Reassuring an older patient because tremor and anxiety are absent despite new atrial fibrillation.
07
Adjusting early treatment solely to TSH while free hormones are already falling.