Anesthetic Considerations in Hypothyroidism 

🧪 Classification of Hypothyroidism

Type

Description

Common Causes

Primary

Thyroid gland pathology

Hashimoto’s, post-thyroidectomy, radioiodine

Secondary

Pituitary failure → ↓TSH

Pituitary tumors, surgery

Tertiary

Hypothalamic failure → ↓TRH

Rare

Subclinical

↑TSH, Normal T3/T4

Early stage


🧬 Clinical Features

System

Manifestations

CVS

Bradycardia, ↓ CO, pericardial effusion, hypotension

Resp

↓ ventilatory drive, hypoventilation, OSA

Neuro

Lethargy, confusion, depression

GI

Hypomotility, constipation

Heme

Anemia, coagulopathy

Skin

Dry, cold intolerance

Other

Hoarseness, weight gain, myxedema, delayed gastric emptying


🧪 Investigations

  • TSH: Elevated in primary hypothyroidism
  • Free T3, T4: Decreased in overt disease
  • ECG: Bradycardia, low-voltage complexes
  • Echocardiography: Pericardial effusion
  • ABG: May show hypercapnia, hypoxemia
  • Electrolytes: Hyponatremia (↓ free water clearance)


💊 Medical Optimization Before Anesthesia

Goal

Action

Normalize thyroid levels

Levothyroxine (start low, go slow)

Correct electrolytes

Especially hyponatremia

Treat anemia

Iron, folate, B12 if needed

Consider steroids

If secondary hypothyroidism or adrenal insufficiency suspected


⚠️ Indications for Delaying Elective Surgery

  • Severe symptoms (myxedema)
  • Untreated hypothyroidism with major systemic compromise
  • Recent onset of angina or pericardial effusion
  • TSH > 50 with abnormal T4 ??

🔹 Minor surgeries may proceed in mild/subclinical cases, with precautions.


💉 Anesthetic Implications of Hypothyroidism

General Concerns

  • ↓ MAC requirement (↓ CNS activity)
  • ↑ Sensitivity to sedatives, opioids, muscle relaxants
  • ↓ hepatic metabolism & renal excretion → prolonged drug action
  • Risk of delayed emergence, hypoventilation, hypothermia


🔄 Preoperative Considerations

  • Assess severity & treatment status
  • Optimize thyroid function
  • Assess for:
    • Pericardial effusion
    • Anemia
    • Electrolyte imbalance
    • OSA symptoms
  • Avoid sedatives & opioids premed if uncorrected
  • Plan active warming strategies


🛌 Induction & Maintenance

Drug Class

Considerations

Induction agents

Propofol or Etomidate (↓ dose), avoid thiopentone in severe bradycardia

Opioids

Use short-acting (fentanyl, remifentanil) in titrated doses

NMBA

Use non-depolarizers with careful monitoring (prolonged action)

Inhalational agents

↓ MAC needed; Sevoflurane preferred

Vasopressors

Phenylephrine, ephedrine in bradycardia

Fluids

Avoid overload due to pericardial effusion risk


💥 Special Caution: Myxedema Coma

  • Medical emergency: severe untreated hypothyroidism
  • Symptoms:
    • Hypothermia
    • Respiratory failure
    • Bradycardia, hypotension
    • Seizures, coma

Management:

  • ICU care
  • IV levothyroxine + corticosteroids
  • Ventilatory and hemodynamic support
  • Active warming


🛌 Regional Anesthesia Considerations

Feature

Hypothyroid Patients

Sensitivity

May have prolonged block

Spinal/Epidural

Generally safe if cardiovascular function stable

Avoid large blocks

If unoptimized (risk of hypotension)

Preferred in mild/moderate

Especially for superficial surgeries


🩺 Postoperative Considerations

  • Close respiratory monitoring – risk of hypoventilation
  • Avoid sedatives or opioids unless fully recovered
  • Maintain normothermia
  • Monitor glucose, sodium, and BP
  • Delayed recovery from anesthesia is common
  • Resume thyroid hormone ASAP


🎯 Viva Triggers

Question

Answer

Why avoid thiopentone?

Myocardial depression, risk of prolonged effect

Why reduce MAC in hypothyroidism?

Decreased CNS metabolism

Precautions in muscle relaxant use?

Prolonged duration due to ↓ metabolism

What is myxedema coma?

Emergency with hypothermia, coma, hypotension

How to manage OSA in hypothyroid?

Pre-op screening, minimize opioids, CPAP if needed


✅ MCQ Nuggets

  1. Best induction agent in severe hypothyroidism?
    → Etomidate
  2. Myxedema coma first-line therapy?
    → IV levothyroxine + hydrocortisone
  3. Thyroid hormone effects on MAC?
    → ↓ MAC in hypothyroid, ↑ MAC in hyperthyroid
  4. Respiratory effect of hypothyroidism?
    → Hypoventilation due to ↓ drive
  5. Thyroid status with pericardial effusion?
    → Severe hypothyroidism