- Primary thyroid disease (95%)
- Autoimmune (Hashimoto's) disease
- Iatrogenic e.g. post-thyroidectomy, inadequate thyroid hormone replacement
- Due to dietary iodine deficiency
- Secondary hypothyroidism due to pituitary disease
Hypothyroidism
Hypothyroidism
The curriculum asks for knowledge of 'endocrine abnormalities of significance to anaesthesia e.g. hypothyroidism'.
Thyroid surgery has appeared in multiple CRQs; 2018, 2020 and 2022.
Resources
- A disease characterised by insufficient thyroid hormone levels
- Prevalence 5 in 1,000 in iodine-sufficient areas, but higher in areas where iodine deficiency is present
- Constitutional symptoms associated with a reduction in basal metabolic rate:
- Weight gain
- Cold intolerance and low body temperature
- Fatigue
- Lethargy
- Depression
- Cardiorespiratory slowing:
- Low respiratory rate
- Depressed responses to hypoxaemia and hypercarbia
- Bradycardia
- Reduced cardiac output due to both bradycardia and negative inotropy
- Autonomic instability
- Pericardial effusions
- Anaemia
- Hypoglycaemia
- Hyponatraemia
- Adrenal insufficiency from cortical atrophy
- Peri-orbital swelling
- Goitre
- Pre-tibial myxoedema
Potential for difficult airway
- Goitre ± anatomical deviation ± airway obstruction
- Anterior mediastinal mass
- Recurrent laryngeal nerve involvement
- Potential for prior neck irradiation to treat (thyroid) cancer
- Obesity
- Laryngeal or pharyngeal oedema
- Macroglossia
Investigations
- Low serum T3/T4
- Elevated serum TSH
- Treat underlying cause
- Replacement of thyroid hormones with thyroxine
- Most patients are on oral T4 replacement therapy (half-life 7 days)
- If urgent replacement is needed than IV T4 is available (half-life 1.5 days)
Perioperative management of the patient with hypothyroidism
History and examination
- Establish disease aetiology, progress and treatments to date
- Elicit features of cardiorespiratory involvement including functional capacity
Investigations
- Bloods
- FBC - anaemia may be present
- TFTs
- Ensure euthyroid prior to surgery
- Certainly avoid surgery in severe hypothyroidism (T4 < 1μg/dL)
- U&E - may have hyponatraemia
- Calcium
- Glucose - may have hypoglycaemia
- 12-lead ECG
- Consider TTE if concerns over cardiac function
Airway assessment
- Patients often have macroglossia, pharyngeal and/or laryngeal oedema and obesity
- Review pre-operative CT scans to assess the degree and extent of tracheal compression from goitre
- Consider FNE if concerns re: laryngoscopic view
Monitoring and access
- AAGBI
- Arterial line if concerns re: cardiac function
Anaesthetic technique
- Regional anaesthetic techniques may be preferable to avoid the effects of GA on already-depressed respiratory and cardiac systems
- If a GA technique is used:
- MAC is reduced
- Positive inotropy may be required as hypotension may be related to cardiac depression as well as the effects of GA
- Patients may demonstrate delayed emergence
Care bundle
- Maintain normothermia; prone to hypothermia
- Monitor for hypoglycaemia
- VTE prophylaxis as standard
- Antibiotic prophylaxis as standard
- Avoid long-acting sedative/opioid drugs where possible to reduce effects on an already-depressed respiratory drive
- A rare complication of hypothyroidism which carries a high mortality (>50%)
- May be triggered by surgical stress, or infection
Clinical features
- Respiratory depression
- Cardiovascular instability; bradycardia | hypotension | cardiac failure
- Depressed conscious level
- Hypothermia
- Metabolic derangement: hypoglycaemia, hyponatraemia
Management
- Supportive therapy e.g. passive rewarming, pacing, inotropic support, ventilatory support
- Thyroid hormone replacement
- IV liothyronine (T3) e.g. 0.2μg/kg QDS
- IV levothyroxine (T4) e.g. 200-500μg IV over 5-10mins followed by 50-100μg daily
- Steroid cover due to impaired ACTH response e.g. 100mg hydrocortisone IV stat then 25mg QDS