Thyroid. Anesthetic implication of hypo and hyperthyroidism. Thyroid function test. Thyrotoxicosis. Inter- Operative. Management of... Hypothyroidism and hyperthyroidism. Anesthetic management.

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Thyroid Disease: Anesthetic Implications, Thyroid Function Tests, Thyrotoxicosis, and Perioperative Management

Primary source: Barash, Cullen, and Stoelting's Clinical Anesthesia, 9e (Chapter 47); supplemented by Quick Compendium of Clinical Pathology, 5e

1. Thyroid Physiology (Relevant to Anesthesia)

The thyroid secretes T4 (prohormone, 80-100 μg/d, t½ ~7 days) and T3 (active hormone, t½ 24-30 hours). About 80% of circulating T3 comes from peripheral deiodination of T4. T4 is also metabolized to inactive reverse T3 (rT3).

Why it matters to the anesthesiologist: Thyroid hormone increases β-adrenergic receptor number and intrinsic myocardial contractility, amplifies catecholamine responses, and regulates metabolic rate across virtually every organ. Abnormalities in thyroid status therefore have profound cardiovascular and metabolic consequences intraoperatively.

Barash, 9e, p. 3987

2. Thyroid Function Tests

Standard Tests and Interpretation

Test	Hyperthyroidism	Primary Hypothyroidism	Secondary Hypothyroidism	Non-thyroidal Illness
TSH	Low/normal	Elevated (>20 μIU/mL)	Low	Normal
Total T4	Elevated	Low	Low	Normal
Total T3	Elevated	Low or normal	Low	Low
Free T4	Elevated	Low	Low	Normal-low
Thyroid Hormone Binding Ratio (THBR)	Elevated	Low	Low	Normal

Normal TSH: 0.4-4.5 μIU/mL
T4 normal: 5-12 μg/dL; T3 normal: 60-180 ng/dL
TSH is the first-line screening test - it is the most sensitive and specific
Barash, 9e, pp. 3988-3990; Quick Compendium of Clinical Pathology, 5e

Important Caveats for Total T4

Total T4 is affected by changes in Thyroxine-Binding Globulin (TBG):

TBG increased by: pregnancy, oral contraceptives, estrogens, acute hepatitis, opioids, clofibrate → falsely elevated total T4 (euthyroid patient appears hyperthyroid)
TBG decreased by: hypoproteinemia, androgens, cortisol → falsely low total T4
Therefore, always measure free T4 alongside total T4

T3 Resin Uptake (T3RU) / Free T4 Index

T3RU is inversely proportional to available TBG binding sites. Free T4 index = T3RU × total T4. This older test is now largely replaced by direct free T4/T3 assays.

Radioactive Iodine Uptake

Elevated in hyperthyroidism (except thyroiditis, where it is low/absent)
"Hot" nodules: rarely malignant
"Cold" nodules: may be malignant or benign
Barash, 9e, p. 3990

3. Hyperthyroidism

Causes

Graves disease (most common) - diffuse goiter + ophthalmopathy + dermopathy + thyroid-stimulating antibody
Toxic multinodular goiter
Thyroid adenoma
Thyroiditis (subacute, Hashimoto)
Pregnancy, amiodarone (iodine-induced - Jod-Basedow phenomenon), contrast media
TSH-secreting pituitary adenoma (rare)

Clinical Features

Weight loss, diarrhea, muscle weakness, warm/moist skin, heat intolerance, nervousness
Cardiovascular: ↑ LV contractility and ejection fraction, tachycardia, ↑ systolic BP, ↓ diastolic BP, ↑ myocardial O2 consumption, ↓ vascular resistance
Atrial fibrillation, hypercalcemia, thrombocytopenia, mild anemia
Elderly may present with apathetic hyperthyroidism (depression, withdrawal, without classic signs)

4. Preoperative Management of Hyperthyroidism

Goal: Render the patient euthyroid before elective surgery

Drug	Mechanism	Notes
Propylthiouracil (PTU)	Inhibits organification + peripheral T4→T3 conversion	First choice; 6-8 weeks to euthyroid state
Methimazole	Inhibits organification	Also 6-8 weeks
Propranolol	β-blockade, also ↓ T4→T3 conversion over 1-2 weeks	Treat CV symptoms; reduce HR <90 bpm
Potassium iodide (Lugol's)	Wolff-Chaikoff effect - inhibits organification + release	2-5 drops q8h; use AFTER antithyroid drugs (iodide alone may worsen thyrotoxicosis)
Dexamethasone 8-12 mg/d	↓ thyroid hormone secretion + peripheral T4→T3 conversion	Used in severe thyrotoxicosis

Preoperative preparation requires 7-14 days (with PTU + iodide combination)
Antithyroid drugs are continued through the morning of surgery
Barash, 9e, pp. 3992-3993

5. Anesthetic Management - Hyperthyroidism

Intraoperative Goals

Achieve a depth of anesthesia that prevents exaggerated sympathetic response to surgical stimulation
Avoid agents that stimulate the sympathetic nervous system

Drug Considerations

Drug	Recommendation	Reason
Ketamine	AVOID - even if clinically euthyroid	Stimulates sympathetic nervous system
β-blockers	Use liberally	Control HR, attenuate adrenergic manifestations
Vasopressors	Use direct-acting agents (phenylephrine, norepinephrine)	Avoid indirect agents that cause catecholamine release
Epinephrine in regional blocks	AVOID	Risk of exaggerated sympathoadrenal response
Muscle relaxants	Reduce initial dose, use twitch monitor	Increased incidence of myasthenia gravis in hyperthyroid patients
Volatile agents	MAC is NOT affected by hyperthyroidism	Can use normally

Regional Anesthesia

Excellent alternative when appropriate
Epinephrine-containing solutions must be avoided

Hypotension Management

Treat with direct-acting vasopressors (not indirect-acting ones like ephedrine which releases endogenous catecholamines)
Barash, 9e, p. 3994

6. Thyrotoxicosis and Thyroid Storm

Risk of Thyroid Storm

The major risk of anesthesia in a poorly controlled thyrotoxic patient is thyroid storm - a life-threatening exacerbation that develops most commonly in undiagnosed/untreated patients due to the stress of surgery or non-thyroidal illness.

Clinical Features

Hyperthermia, tachycardia, dysrhythmias, myocardial ischemia, congestive heart failure, agitation, confusion

Differential Diagnosis (Must exclude)

Pheochromocytoma
Malignant hyperthermia
Light anesthesia (inadequate depth)

Emergency Surgery in a Thyrotoxic Patient

β-adrenergic blockade to achieve HR < 90 bpm
β-blockers do NOT prevent thyroid storm - they only control symptoms

Management of Thyroid Storm (Table 47-3)

Intervention	Regimen
IV fluids	Aggressive resuscitation
Sodium iodide	250 mg PO/IV q6h
Propylthiouracil	200-400 mg PO/NGT q6h
Hydrocortisone	50-100 mg IV q6h
Propranolol	10-40 mg PO q4-6h OR esmolol infusion (to treat hyperadrenergic signs)
Cooling measures	Cooling blankets + acetaminophen + meperidine 25-50 mg IV q4-6h (to prevent shivering)
Cholestyramine	4 g PO q6h (adjunctive - interrupts enterohepatic recirculation of T4)

Barash, 9e, p. 3994

7. Hypothyroidism

Epidemiology and Causes

Affects 0.3-5% of adults. Primary hypothyroidism (95% of cases) = thyroid gland fails despite adequate TSH.

Causes:

Autoimmune (Hashimoto thyroiditis)
Previous radioiodine therapy or neck irradiation
Surgical removal
Medications (PTU, methimazole, iodine excess)
Infiltrative disorders (amyloidosis, sarcoidosis)
Secondary/tertiary: hypopituitarism, hypothalamic disease

Clinical Features

Cardiovascular (most relevant to anesthesia):

Bradycardia, decreased cardiac output, increased peripheral resistance
Low voltage ECG (cholesterol-rich pericardial effusion)
Heart failure only if coexisting cardiac disease

Other:

Lethargy, slow mental function, cold intolerance
Ventilatory responses to hypoxia and hypercapnia are depressed - potentiated by sedatives, opioids, and general anesthesia
Anemia, coagulopathy, hypothermia, sleep apnea
Hyponatremia (impaired free water clearance)
Decreased GI motility → postoperative ileus
Adrenal depression in longstanding/severe disease (blunted stress response)

8. Preoperative Assessment - Hypothyroidism

Severity	Recommendation
Mild/moderate	No compelling reason to postpone surgery. Minor ↑ intraoperative hypotension and postoperative GI/neuropsychiatric issues - not clinically significant
Severe hypothyroidism	Postpone elective surgery until at least partially treated
Myxedema coma	Only lifesaving surgery - aggressive medical therapy first
Concurrent CAD	Weigh thyroid replacement risk (may precipitate ischemia) vs. benefits; delay replacement until after coronary revascularization in unstable patients

Barash, 9e, p. 3998

9. Anesthetic Management - Hypothyroidism

Drug Considerations

Drug	Recommendation	Reason
Ketamine	Preferred induction agent	Stimulates sympathetic NS - counteracts ↓ cardiac output
Volatile agents	MAC is essentially unchanged	Use normal doses
IV agents	Can be used safely
Regional anesthesia	Excellent choice	Provided intravascular volume is well maintained
Sedatives/opioids	Use cautiously, reduce doses	Depress already compromised ventilatory responses

Intraoperative Monitoring Priorities

Early recognition of hypotension (↓ cardiac output, ↓ peripheral vascular reserve)
Congestive heart failure
Hypothermia - scrupulous temperature maintenance; hypothyroid patients are already cold-intolerant

Management of Myxedema Coma (Table 47-5)

Intervention	Regimen
Tracheal intubation + controlled ventilation	As needed (respiratory failure expected)
Levothyroxine (T4)	200-300 μg IV over 5-10 min, then 100 μg IV q24h
Or T3	More rapid onset alternative
Hydrocortisone	100 mg IV loading, then 25 mg IV q6h
Fluid and electrolyte replacement	Guided by serum electrolytes
Temperature conservation	Cover patient; no warming blankets (may cause vasodilation → cardiovascular collapse)

Mortality of myxedema coma: 25-50% - aggressive treatment mandatory

Barash, 9e, p. 3999

10. Anesthesia for Thyroid Surgery (Specific Considerations)

Airway Management

Difficult intubation incidence: 5-8% during goiter surgery
Thyroid cancer ↑ difficult intubation risk; goiter SIZE alone is not predictive
Large goiters with airway obstruction or tracheal deviation: awake intubation is warranted
Large substernal goiters can behave like anterior mediastinal masses → intrathoracic airway obstruction after induction
Always review CT/MRI prior to surgery
Minimally invasive transoral approach: requires nasal intubation

Nerve Integrity Monitoring (NIM)

NIM endotracheal tube (Medtronic Xomed) has electrodes contacting vocal cords to monitor RLN electromyographically
Must avoid muscle relaxants and topical laryngeal anesthesia when NIM tube is in use
Succinylcholine or small dose rocuronium may be used for intubation (ensure proper repositioning after neck manipulation)

Post-Thyroidectomy Complications

Complication	Features	Management
RLN injury (unilateral)	Hoarseness, unilateral cord paralysis	Usually transient; assess phonation pre/postop
RLN injury (bilateral)	Stridor, aphonia	Immediate reintubation
Hematoma	Tracheal compression	Surgical evacuation
Hypoparathyroidism	Hypocalcemia within 24-96 hours; laryngeal stridor → laryngospasm (first sign of hypocalcemic tetany)	IV calcium chloride or gluconate; monitor Mg
Pneumothorax	In substernal goiter with tracheomalacia	Manage accordingly
Tracheomalacia	Tracheal collapse after long-standing goiter	May require emergency reintubation

Quick Summary Table: Key Anesthetic Differences

Parameter	Hyperthyroidism	Hypothyroidism
Preop target	Euthyroid state (6-8 weeks)	Treat severe disease; mild can proceed
Induction agent	Avoid ketamine	Prefer ketamine
MAC	Unchanged	Unchanged (minimal change)
Muscle relaxants	↓ dose (myasthenia risk)	Standard
Vasopressor	Direct-acting only	Standard
Regional anesthesia	Yes, no epi	Yes, if volume maintained
Main intraop risk	Thyroid storm	Hypotension, hypothermia, slow awakening
Ventilatory concern	Hypermetabolic state	Blunted hypoxic/hypercapnic drive
Temperature	Fever/heat intolerance	Hypothermia (no warming blankets in myxedema)

Sources: Barash, Cullen, and Stoelting's Clinical Anesthesia, 9e (Chapter 47); Quick Compendium of Clinical Pathology, 5e

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