What considerations are relevant to administering anesthesia to a patient with hypothyroidism?

Anesthetic Considerations for Hypothyroid Patients

Primary Recommendation

Elective surgery should be postponed until hypothyroidism is adequately treated and euthyroidism is documented by serum TSH measurement, as untreated hypothyroidism significantly increases perioperative complications including intraoperative hypotension, postoperative heart failure, gastrointestinal complications, and neuropsychiatric disturbances. 1, 2, 3

Critical Preoperative Assessment

Thyroid Status Evaluation

• Measure serum TSH and free T4 preoperatively to distinguish true hypothyroidism from euthyroid sick syndrome, as the latter does not require thyroid hormone replacement and treatment may be harmful 1
• Document euthyroidism (TSH 0.5-2.0 mIU/L) before proceeding with elective surgery 4
• Exception: Patients with coronary artery disease requiring revascularization should undergo cardiac intervention first, as preoperative thyroid hormone therapy could exacerbate myocardial ischemia 1

Assess for Central Hypothyroidism

• If TSH is low or inappropriately normal with low free T4, evaluate for hypophysitis or secondary hypothyroidism 5
• Critical: Initiate hydrocortisone BEFORE thyroid hormone replacement in central hypothyroidism, as thyroid hormone accelerates cortisol clearance and can precipitate adrenal crisis 5
• Evaluate morning ACTH and cortisol if central hypothyroidism suspected 5

Emergency Surgery in Untreated Hypothyroidism

Preoperative Preparation

• Administer thyroid hormone supplementation before emergency surgery whenever possible 3
• For severe hypothyroidism, consider intravenous levothyroxine in consultation with endocrinology 5
• If myxedema coma suspected (bradycardia, hypothermia, altered mental status), give hydrocortisone before thyroid hormone to avoid precipitating adrenal crisis 5

Anticipated Complications

The following complications occur more frequently in hypothyroid patients and require proactive management:

Cardiovascular:

• Intraoperative hypotension (61% vs 30% in controls) due to decreased myocardial contractility and abnormal baroreceptor function 2, 3
• Postoperative heart failure after cardiac surgery (29% vs 6% in controls) 2
• Prepare vasopressor support and avoid aggressive fluid administration due to impaired free water excretion 3

Respiratory:

• Decreased hypoxic and hypercapnic ventilatory responses increase risk of respiratory depression 3
• Monitor closely for delayed anesthetic recovery and hypoventilation 3

Metabolic:

• Hypothermia is common—use active warming measures intraoperatively and postoperatively 3
• Hyponatremia from impaired free water excretion—monitor electrolytes closely 3
• Hypoglycemia risk—check glucose perioperatively 3

Gastrointestinal:

• Postoperative gastrointestinal complications (19% vs 1% in controls), including ileus 2

Neuropsychiatric:

• Postoperative neuropsychiatric complications (38% vs 18% in controls), including delirium 2

Infection:

• Blunted fever response despite infection (35% vs 79% fever with infection)—do not rely on fever as infection marker 2

Anesthetic Drug Considerations

Reduced Drug Metabolism

• Hypothyroidism causes impaired hepatic drug metabolism 3
• Reduce doses of sedatives, opioids, and anesthetic agents to avoid prolonged effects 3
• Anticipate delayed emergence from anesthesia 3

Volume Status

• Plasma volume is reduced in hypothyroidism 3
• Titrate fluid administration carefully, avoiding both hypovolemia and fluid overload 3

Postoperative Management

Monitoring Priorities

• Continuous temperature monitoring and active rewarming as needed 3
• Serial electrolyte monitoring, particularly sodium 3
• Vigilant infection surveillance despite potentially absent fever 2
• Extended respiratory monitoring for hypoventilation 3

Thyroid Hormone Continuation

• Resume or initiate levothyroxine as soon as enteral route available 5
• For central hypothyroidism, ensure adequate corticosteroid replacement is established first 5

Common Pitfalls to Avoid

1. Do not proceed with elective surgery in untreated hypothyroidism—most reported complications occur in patients with unrecognized disease 3
2. Do not treat euthyroid sick syndrome with thyroid hormone—distinguish this from true hypothyroidism using TSH and free T4 1
3. Do not start thyroid hormone before corticosteroids in central hypothyroidism—this can precipitate life-threatening adrenal crisis 5
4. Do not rely on fever to detect infection—hypothyroid patients have blunted febrile responses 2
5. Do not use standard anesthetic drug doses—reduce doses due to impaired metabolism 3