Hyperthyroidism

Although the curriculum doesn't explicitly mention hyperthyroidism, it's presumed to be fair game as a topic.

Thyroid surgery has appeared in multiple CRQs; 2018, 2020 and 2022.

Resources

A multisystem disorder arising due to the excessive production of thyroid hormones
10x more common in females

Autoimmune (Graves') disease (60-90%)

IgG auto-antibodies which mimic TSH

Toxic multinodular goitre

Neoplasms:

Thyroid: functional thyroid adenoma
Pituitary: TSH-secreting adenoma

Drug-associated

Excessive treatment with levothyroxine
Amiodarone
Exogeneous iodine

Post-irradiation thyroiditis
Molar pregnancy (rare)

Clinical features arise due to an elevated basal metabolic rate, including a raised VO₂ and VCO₂

Cardiovascular

Tachycardia
Arrhythmias e.g. AF
Palpitations
High-output cardiac failure
Ischaemic cardiac events

Neurological

Agitation
Restlessness
Anxiety
Fine tremor
Proximal myopathy

Gastrointestinal

Weight loss
Increased appetite
Diarrhoea

Constitutional

Heat intolerance
Sweating
Hair loss (esp. outer 1/3^rd of the eyebrows)
Palmar erythema
Oligomenorrhoea

Ocular

Exophthalmos
Dry eyes
Red eyes
Visual disturbance

Classically there is a raised serum T₃/T₄
Serum TSH is correspondingly low

Patients may have a normal T₃/T₄ but still low TSH
This is termed sub-clinical hyperthyroidism and in the absence of clinical features can proceed with surgery

Management

β-blockade is used as it

Reduces the peripheral conversion of T₄ to T₃
Reduces the cardiovascular effects of thyroid hormones

Pharmacological management of excessive thyroid hormones

Carbimazole
Propylthiouracil

Interventional

Radioactive iodine
Thyroidectomy

Perioperative management of the patient with hyperthyroidism

Anaesthetic morbidity relates to:

Cardiac risk
Risk of perioeprative thyroid storm in the non-euthyroid patient

History and examination

Establish aetiology of thyroid disease, course and therapies to date
Thorough airway assessment; may have difficult airway due to goitre

Investigations

Bloods

FBC - both carbimazole and propylthiouracil can cause agranulocytosis
Calcium - as risk of post-operative hypocalcaemia due to loss of parathyroid glands
Group and saves with ability to rapidly cross-match, as high propensity for blood loss
TFTs

Ensure euthyroid prior to surgery to reduce risk of thyroid storm
May require up to 8 weeks' treatment with PTU/ oral carbimazole
May require Lugol's iodine (I₁₃₁) 7-10 days pre-operatively to reduce thyroid gland vascularity and risk of thyroid storm

12-lead ECG

Target resting heart rate <85bpm
May have sinus bradycardia from β-blocker therapy
May require IV β-blocker therapy to control cardiovascular symptoms prior to emergency surgery

Consider TTE or other cardiac investigations if evidence of cardiac failure, ischaemic heart disease or other dysrhythmias

Airway assessment

CXR may demonstrate tracheal narrowing or deviation
Review CT images to assess degree of tracheal compression and/or retrosternal extension
FNE if concerns about visualising the larynx at laryngoscopy

Monitoring and access

AAGBI
Consider arterial line

Anaesthetic technique

Aim to minimise sympathetic response to stimuli, which may be exaggerated and precipitate dysrhythmia

E.g. use remifentanil TCI to suppress laryngoscopic pressor response

Use direct vasopressors (e.g. phenylephrine) preferentially over indirect sympathomimetics (ephedrine)
Beware using anticholinergic medication
Consider avoiding other sympathetic stimulants e.g. ketamine

Tend to have altered anaesthetic requirements

Increased MAC
Higher volume of distribution and clearance of propofol; if TIVA/TCI is used may need a higher effect site

Nerve monitoring may be required, so use judicious amounts of NMBA or avoid altogether

Care bundle

Appropriate eye protection, especially if exophthalmos is present
Temperature monitoring

Multi-modal analgesia to reduce stress response and sympathetic response to analgesia

A rare but potentially life-threatening acute complication in patient's with hyperthyroidism

Epidemiology

Incidence <1 per 100,000/yr
Mortality 10-30%
Lack of evidence about the risk of perioperative thyroid storm, but it does appear to occur:

Regardless of the preoperative treatment used
In those who are both hyperthyroid and euthyroid (BJA, 2021)

Pathophysiology

Occurs due to excessive thyroid hormone release, which itself arises due to:

Handling of the thyroid tissue intra-operatively
Surgical stress
Infection
Pregnancy, pre-eclampsia or hyperemesis
Trauma including burns
Acute medical events e.g. MI, PE, DKA
Abrupt cessation of anti-thyroid medication

Clinical features

Presents 6-24hrs post-operatively
Symptoms appear similar to MH although there is no muscle rigidity in thyroid storm

Cardiovascular

Tachycardia
Hypertension
Dysrhythmia
Acute cardiac failure

Neurological

Agitation
Delirium
Psychosis
Seizures

Hyperpyrexia
Nausea and vomiting
Diarrhoea
Abdominal pain
Liver failure

Diagnosis

Diagnosis may be difficult in the perioperative period owing to overlapping features with other life-threatening conditions e.g. septic shock, MH
Scoring systems to aid diagnosis include:

Burch and Wartofsky Point Scale (1993)
Akamizu Criteria (2012)

Management

Supportive therapy including passive cooling, anti-pyretics e.g. paracetamol and sedation

Pharmacotherapy

IV beta blockers e.g. esmolol 250-500μg/kg bolus or 50-200 μg/kg/min infusion
Use digoxin or beta blockers to control dysrhythmia rather than amiodarone
Magnesium
Hydrocortisone due to inappropriate ACTH response e.g. 100-200mg TDS
Treat convulsions using standard therapeutic ladder
Anti-thyroid medication e.g. propylthiouracil 500-1,000mg stat then 200-400 mg PO/NG/PR 4-6hrly
Potassium or sodium iodide to stop synthesis and release of new thyroid hormones
Dantrolene has been successfully used in cases whether the diagnosis was mistaken for MH

Interventional

Plasma exchange
Urgent thyroidectomy
Neuraxial block to T4 level