Perioperative Management of Hyperthyroidism in Anesthesia

1. Preoperative Assessment

• TSH (suppressed in hyperthyroidism)
• Free T3 and free T4 (elevated)
• ECG (assess for AF, rate, ischemia)
• If known thyroid disease on stable therapy, no repeat TFTs needed if euthyroid within 6 months

• Goiter incidence of difficult intubation: 5-8%
• Thyroid cancer independently increases difficult intubation risk (not size alone)
• Large goiters with stridor, tracheal deviation, or substernal extension: plan awake intubation
• Substernal goiters act as anterior mediastinal masses - can cause intrathoracic airway obstruction post-induction
• Review CT/MRI pre-op for any significant goiter

2. Operative Timing - Core Principle

Elective surgery must be postponed until the patient is rendered euthyroid.

3. Preoperative Medical Optimization

4. Intraoperative Anesthetic Management

5. Airway Management for Thyroidectomy

• Preferred: General endotracheal anesthesia
• LMA use is increasing - allows real-time vocal cord visualization with spontaneous ventilation
• Limited thyroidectomy: may use bilateral superficial cervical plexus block
• Minimize for transoral approach: nasal intubation required
• Robot-assisted transaxillary procedures are increasing
• Have video laryngoscopy and awake FOB plan ready for anticipated difficult airway

• NIM (Nerve Integrity Monitor) endotracheal tube (Medtronic Xomed) - electrodes embedded in shaft above cuff, provides EMG response to nerve stimulation
• Surgeon may use nerve stimulator with direct palpation of laryngeal muscles

6. Postoperative Complications (Thyroidectomy-Specific)

7. Thyroid Storm - Recognition & Emergency Management

Treatment Protocol (Barash Table 47-3 + Rosen's EM)

8. Special Populations

• Pregnancy: Radioactive iodine is contraindicated (crosses placenta, destroys fetal thyroid). PTU preferred in first trimester; methimazole in second/third.
• Myasthenia gravis: Screen for it - incidence is increased in hyperthyroid patients. Reduce initial NMB dose.
• Heart failure: Beta blockade may not correct LV dysfunction from hyperthyroidism. Rate control may improve AF-related failure, but intrinsic LV dysfunction needs targeted therapy.
• Asthma/COPD: Use beta-1 selective agents (esmolol, metoprolol) instead of propranolol for rate control.

• Barash, Cullen & Stoelting's Clinical Anesthesia, 9e - Chapters 13, 47 (Thyroid and Parathyroid Diseases; Anesthesia for Thyroid Surgery)
• Miller's Anesthesia, 10e - Chapter: Thyroid Disease (Perioperative Considerations)
• Fischer's Mastery of Surgery, 8e - Thyroid Gland
• Rosen's Emergency Medicine, Concepts and Clinical Practice - Management of Thyrotoxicosis (Box 117.3)

Perioperative Management of Hypothyroidism in Anesthesia

1. Background & Epidemiology

2. Common Causes (Table 47-4, Barash)

3. Clinical Features Relevant to Anesthesia

• Bradycardia, decreased cardiac output, increased peripheral vascular resistance
• Low-voltage ECG (cholesterol-rich pericardial effusion)
• Heart failure is rare unless coexisting cardiac disease is present
• Angina pectoris is unusual in hypothyroidism, but can appear when thyroid hormone replacement is initiated (important periop consideration)
• Diastolic hypertension common

• Depressed ventilatory response to hypoxia and hypercapnia - potentiated by sedatives, opioids, and general anesthesia
• Sleep apnea common
• Postoperative ventilatory failure is rare unless coexisting lung disease, obesity, or myxedema coma is present

• Lethargy, cold intolerance, slow mentation, dry skin, weight gain
• Anemia, coagulopathy
• Hyponatremia (impaired renal free water clearance)
• Hypothermia (impaired thermogenesis)
• Decreased GI motility - contributes to postoperative ileus
• Goiter - dysphagia, dyspnea, stridor (airway concern)
• In longstanding/severe disease: blunted stress response, adrenal depression

4. Preoperative Assessment

• TSH - best single test for hypothyroidism (elevated in primary)
• Free T4 - low in overt disease
• ECG (look for low voltage, bradycardia, conduction defects)
• Electrolytes (hyponatremia)
• CBC (anemia)
• Coagulation screen
• Chest X-ray or CT/MRI if goiter present (assess tracheal deviation, mediastinal involvement)

Miller's Anesthesia key principle: "If a patient has moderate or worse hypothyroidism (elevated TSH and low free T4, with or without symptoms), elective surgery should be postponed until the individual is euthyroid."

5. Preoperative Optimization

• Continue levothyroxine on the morning of surgery (all thyroid replacement therapy is continued)
• Target euthyroid state before elective surgery - this typically takes weeks with oral levothyroxine (T4 plasma half-life ~7 days)
• If surgery is urgent in an overt hypothyroid patient: IV levothyroxine can be given; consult endocrinology
• In patients with hypothyroidism + symptomatic coronary artery disease: thyroid replacement may precipitate myocardial ischemia. In unstable cardiac ischemia, delay thyroid replacement until after coronary revascularization

6. Intraoperative Anesthetic Management

• Mild-to-moderate hypothyroidism: no significant increase in serious perioperative complications under general anesthesia; most anesthetic medications can be used without difficulty
• Severe hypothyroidism: high-risk - proceed only for urgent/emergent indications

• Early recognition of hypotension, CHF, and hypothermia
• Scrupulous temperature management - hypothyroid patients cannot thermoregulate adequately
• Use warm IV fluids, warming blankets, maintain OR temperature
• Invasive hemodynamic monitoring if significant cardiac dysfunction

7. Airway Management

• Goiter may cause tracheal deviation or substernal compression - assess with CT/MRI preoperatively
• Plan for possible difficult airway: video laryngoscopy, awake FOB available
• Macroglossia and myxedematous tissue changes can contribute to difficult mask ventilation and intubation
• Post-extubation stridor: assess for tracheomalacia after longstanding goiter

8. Postoperative Considerations

• Prolonged recovery and sedation are possible - monitor closely in PACU
• Risk of postoperative ventilatory failure is low unless coexisting lung disease, obesity, or myxedema coma
• Risk of postoperative ileus - reduced GI motility
• Continue levothyroxine in the postoperative period; if patient is NBM, switch to equivalent IV dose (IV dose = ~75-80% of oral dose, as oral bioavailability is ~70-80%)
• Monitor temperature, glucose, sodium postoperatively

9. Myxedema Coma - Emergency Management

1. Hypothermia
2. Hypoventilation (respiratory depression)
3. Hypotension
4. Hyponatremia
5. Altered consciousness (stupor → coma)

Treatment Protocol (Barash Table 47-5 + Goodman & Gilman):

Critical note on warming: Use passive warming (blankets to conserve body heat) only. Active warming blankets cause peripheral vasodilation, which can precipitate cardiovascular collapse in an already compromised patient.

Aspirin must be avoided - it displaces T4 and T3 from protein binding, worsening thyrotoxicity if correction overshoots. Use acetaminophen for fever.

10. Special Populations

Quick Comparison: Hypothyroidism vs. Hyperthyroidism Periop

• Barash, Cullen & Stoelting's Clinical Anesthesia, 9e - Chapter 47 (Hypothyroidism, Table 47-4, Table 47-5, Treatment and Anesthetic Considerations)
• Miller's Anesthesia, 10e - Thyroid Disease section (Perioperative evaluation and management)
• Goodman & Gilman's Pharmacological Basis of Therapeutics - Myxedema Coma
• Rosen's Emergency Medicine - Hypothyroidism chapter