ECG criteria
- Frequently sinus tachycardia (HR >100 bpm), the degree is variable; normal HR, especially in the elderly, during beta-blocker therapy, or during sleep, does not rule out thyrotoxicosis
- Rhythm is sinus, P wave is positive in II, negative in aVR, preceding each QRS; nonspecific P amplitude variations are possible, with no specific diagnostic criterion
- PQ (PR) interval is generally without specific changes; nonspecific variations within the normal range are possible
- QT may be shortened due to tachycardia; QTc is sometimes shortened, but the result is variable and nonspecific — both normal and prolonged QTc have been described
- ECG changes in voltage and QRS morphology are nonspecific; voltage criteria for hypertrophy (Sokolow–Lyon, Cornell) are not signs of thyrotoxicosis per se
- If arrhythmia is present — atrial fibrillation (absence of organized P waves, f waves may be present, RR intervals are irregularly irregular) or supraventricular extrasystole/tachycardia
- After correction of thyrotoxicosis, tachycardia usually decreases; QT/QTc changes may regress if they were associated with thyrotoxicosis
What it looks like on ECG
The most frequent finding is sinus tachycardia: regular rhythm, P wave positive in II and negative in aVR, heart rate is variable and can exceed 140 bpm in severe thyrotoxicosis. Tachycardia often persists or is out of proportion to the level of exertion, but usually decreases during sleep and at rest — the absence of such a decrease is not a mandatory criterion. A normal heart rate does not rule out thyrotoxicosis, especially in elderly patients, during beta-blocker therapy, or during sleep. QRS is narrow unless there is a concomitant bundle branch block.
The QT interval is mechanically shortened during tachycardia, and the calculated QTc is sometimes at or below the lower limit of normal — this is attributed to accelerated ventricular repolarization under the action of thyroid hormones. However, data on QTc in thyrotoxicosis are heterogeneous: normal and even prolonged values have been described; the result depends on heart rate, electrolytes, concomitant medications, and the chosen formula. At high heart rates, Bazett's formula often overestimates QTc, making Fredericia's formula preferable.
Changes in P wave amplitude and morphology, PQ interval, and QRS voltage and axis in thyrotoxicosis are nonspecific and have no established diagnostic value; identified deviations require separate evaluation (structural pathology, concomitant hypertrophy, block, ischemia).
Approximately 5–15% of patients with overt thyrotoxicosis (more often in the elderly, with prolonged or severe disease — the rate is higher) develop atrial fibrillation: organized P waves are absent, f waves may be detected, RR intervals are irregularly irregular. Atrial tachycardia, atrial flutter, and supraventricular extrasystole are less common.
| Sign | Typical change |
|---|---|
| Rhythm | frequently sinus tachycardia, AF is possible |
| HR | variable, frequently >100 bpm |
| QTc | variable, sometimes shortened |
| QRS | nonspecific changes |
| PQ | generally without specific changes |
In some patients, the ECG may show no specific changes other than tachycardia — this is a nonspecific but frequent finding in overt thyrotoxicosis.
Mechanism
Excess thyroid hormones (T3, T4) increase the sensitivity and density of myocardial β-adrenoreceptors, enhance sinus node automaticity, and accelerate AV conduction, which clinically manifests as tachycardia, often out of proportion to the level of physical exertion. The effect on various ion currents and ventricular repolarization is complex and variable, so QT/QTc changes are inconsistent and nonspecific. An increase in cardiac output and circulating blood volume creates a hyperkinetic circulation type. Structural and electrophysiological changes in the atria (dilation, shortening of the refractory period) in long-standing untreated thyrotoxicosis predispose to atrial fibrillation.
Causes
- Diffuse toxic goiter (Graves' disease) is the most frequent cause of hyperthyroidism
- Toxic thyroid adenoma, multinodular toxic goiter
- Thyrotoxic phase of subacute or autoimmune thyroiditis (thyrotoxicosis without true hyperthyroidism)
- Excessive intake of levothyroxine preparations (exogenous thyrotoxicosis, without hyperthyroidism)
- Iodine-induced thyrotoxicosis
Differential diagnosis
- Inappropriate sinus tachycardia — tachycardia without laboratory signs of thyrotoxicosis, a diagnosis of exclusion.
- Anxiety disorder / autonomic dysfunction — situational tachycardia, TSH and T4 levels are normal.
- Anemia — tachycardia and hyperkinetic circulation type, distinguished by complete blood count data.
- Short QT syndrome — congenital persistent QT shortening without relation to heart rate and without laboratory signs of thyrotoxicosis.
- Sympathomimetic / caffeine overdose — similar tachycardia, distinguished by history and normal TSH/T4 levels.
Clinical significance and management
Persistent or exertion-unproportionate sinus tachycardia without an apparent cause is an indication to investigate thyroid function (TSH, free T4, T3). When atrial fibrillation is detected in a patient without obvious structural heart disease, thyrotoxicosis is included in the standard list of causes to rule out. The physician should assess the risk of thromboembolism (CHA2DS2-VASc score) and control ventricular rate prior to thyroid status correction. In case of hemodynamic instability, immediate electrical cardioversion is performed. In stable persistent AF, elective cardioversion is usually delayed until euthyroidism is achieved, as the recurrence rate is high during thyrotoxicosis; after normalization of thyroid status, it is possible according to standard rhythm control indications, taking into account AF duration and anticoagulation requirements.
Thyroid storm is a life-threatening complication of thyrotoxicosis (rather than its cause) requiring intensive care: β-blockade in the absence of contraindications for rate control, antithyroid drugs, iodine preparations (administered after antithyroid drugs), glucocorticoid, treatment of the triggering factor, and supportive measures. Ventricular rate control is the primary objective in concomitant AF; emergency electrical cardioversion in a storm is indicated only for hemodynamic instability.
After achieving euthyroidism, tachycardia usually decreases; QT/QTc changes may regress if they were specifically related to thyrotoxicosis and no other causes are present. AF may persist even after normalization of thyroid status, especially with a long history of arrhythmia, and require separate rate or rhythm control tactics.
Common interpretation errors
- Interpretation of nonspecific QRS voltage changes as true left ventricular hypertrophy without clinical confirmation.
- Ignoring thyrotoxicosis as a cause of newly diagnosed AF, especially in young patients.
- Calculating QTc in tachycardia without considering the measurement method and formula selection (Fredericia is preferable at high heart rates) and without considering electrolytes and medications.
- Claiming that a normal heart rate rules out thyrotoxicosis, without considering advanced age and beta-blocker therapy.
- Attributing isolated persistent sinus tachycardia solely to anxiety or pain without investigating thyroid function.
1. Which statement regarding heart rate in thyrotoxicosis is correct?
The degree of tachycardia is variable; in the elderly, during sleep, and on beta-blocker therapy, the heart rate may remain normal.
2. Which arrhythmia develops most frequently in long-standing thyrotoxicosis?
Dilation and shortening of the atrial refractory period in thyrotoxicosis predispose to atrial fibrillation.
3. How to correctly characterize QTc changes in hyperthyroidism?
Published data on QTc in thyrotoxicosis are heterogeneous: shortened, normal, and prolonged values are encountered.
Frequently asked questions
Which arrhythmia is most commonly associated with hyperthyroidism?
Most commonly this is sinus tachycardia, and among pathological arrhythmias — atrial fibrillation, especially in patients over 60 years of age and in prolonged or severe thyrotoxicosis. Newly diagnosed AF without an obvious cardiac cause is an indication to check thyroid function.
Does the QT interval shorten in hyperthyroidism?
The QT interval may shorten due to tachycardia, and the QTc sometimes falls at or below the lower limit of normal, but this sign is inconstant and nonspecific — both normal and prolonged QTc values have been described. The result is influenced by heart rate, electrolytes, medications, and the choice of calculation formula.
Is thyrotoxicosis dangerous for the heart?
Prolonged untreated thyrotoxicosis increases the risk of atrial fibrillation, thromboembolic complications, and tachycardiomyopathy with a decrease in myocardial contractility. Thyrotoxic storm is a life-threatening complication of thyrotoxicosis presenting with severe tachyarrhythmia, rather than its cause.
Does the ECG normalize after treatment for hyperthyroidism?
Sinus tachycardia generally decreases as euthyroidism is achieved, and QT/QTc changes may regress if they were specifically caused by thyrotoxicosis. Atrial fibrillation may persist even after normalization of thyroid status, especially with a long history of arrhythmia.
Should the thyroid gland be checked in newly diagnosed AF?
Yes, determination of TSH and free T4 is part of the standard diagnostic minimum for newly diagnosed atrial fibrillation, since thyrotoxicosis is one of the reversible causes of arrhythmia.
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Related terms
More in "Electrolytes and Metabolism"
Sources
- Goldberger's Clinical Electrocardiography, 9th ed.
- Hampton. The ECG in Practice
- Surawicz, Knilans. Chou's Electrocardiography in Clinical Practice
- Orlov V.N. Guide to Electrocardiography
Educational material for medical students and physicians. Does not replace clinical guidelines and physical examination; the ECG must always be interpreted in conjunction with the clinical picture.