TABLE 46-1 EFFECTS OF TRIIODOTHYRONINE ON RECEPTOR CONCENTRATIONS

Increased number of β-receptors
Decreased number of cardiac cholinergic receptors

TABLE 46-2 TESTS OF THYROID FUNCTION

2. Anesthesia for thyroid surgery (subtotal thyroidectomy) is an alternative to prolonged medical therapy. Complications associated with surgery occur more frequently when preoperative preparation is inadequate (Tables 46-4 and 46-5).

a. It is useful to evaluate vocal cord function in the early postoperative period by asking patients to say the letter “e.”

b. Unexpected difficult intubation is increased in the presence of goiter. Inhalation induction or awake fiberoptic intubation should be considered if there is evidence of significant airway obstruction or tracheal deviation or narrowing.

TABLE 46-3 PREPARATION OF HYPERTHYROID PATIENTS

Propylthiouracil: inhibits synthesis and decreases peripheral conversion of T₄ to T₃

Inorganic iodide: inhibits hormone release

β-Adrenergic antagonists: propranolol administered over 12–24 hours decreases the heart rate to <90 beats/min

Iopanoic acid: radiographic contrast agent that decreases peripheral conversion of T₄ to T₃

Glucocorticoids: decrease hormone release and peripheral conversion of T₄ to T₃

T₃ = triiodothyronine; T₄ = thyroxine.

TABLE 46-4 POSSIBLE COMPLICATIONS OF THYROID SURGERY

Thyroid storm: should be distinguished from malignant hyperthermia, pheochromocytoma, and inadequate anesthesia; it most often develops in undiagnosed or untreated hyperthyroid patients because of the stress of surgery

Airway obstruction: diagnosed with CT of the neck

Recurrent laryngeal nerve damage: hoarseness may be present if the damage is unilateral, and aphonia may be present if the damage is bilateral

Hypoparathyroidism: symptoms of hypocalcemia develop within 24 to 48 hours and include laryngospasm

CT = computed tomography.

c. Postoperative airway obstruction caused by hematoma or tracheomalacia may require urgent reintubation of the trachea.

d. Operating on an acutely hyperthyroid patient may provoke thyroid storm.

D. Hypothyroidism

1. Hypothyroidism is a relatively common disease (0.5%–0.8% of the adult population) that results from inadequate circulating levels of T₄, T₃, or both (Table 46-6).

2. Treatment and Anesthetic Considerations

a. No evidence supports postponement of elective surgery (including coronary artery bypass graft surgery) in the presence of mild to moderate hypothyroidism.

TABLE 46-5 MANAGEMENT OF THYROID STORM

IV fluids

Sodium iodide: 250 mg orally or IV every 6 hr

Propylthiouracil: 200–400 mg orally or via a nasogastric tube every 6 hr

Hydrocortisone: 50–100 mg IV every 6 hr

Propranolol: 10–40 mg orally every 4–6 hr or esmolol (titrate)

Cooling blankets and acetaminophen: 12.5 mg IV of meperidine every 4–6 hr may be used to treat or prevent shivering

Digoxin: Congestive heart failure with atrial fibrillation and rapid ventricular response

IV = intravenous.

TABLE 46-6 MANIFESTATIONS OF HYPOTHYROIDISM

Lethargy

Cold intolerance

Decreased cardiac output and heart rate

Peripheral vasoconstriction

Heart failure (unlikely unless coexisting cardiac disease is present)

Decreased platelet adhesiveness

Anemia (GI bleeding)

Impaired renal concentrating ability

Adrenal cortex suppression

Decreased GI motility (may compound the effects of postoperative ileus)

GI = gastrointestinal.

b. No evidence supports the choice of a specific anesthetic technique or selection of drugs for hypothyroid patients, although opioids and volatile anesthetics are often considered to have increased depressant effects in these patients. There appears to be little, if any, decrease in anesthetic requirements as reflected by the minimum alveolar concentration.

c. Meticulous attention must be paid to maintaining body temperature.

3. Myxedema coma is a medical emergency that requires aggressive therapy (Table 46-7).

II. PARATHYROID GLANDS

A. Calcium Physiology. Parathyroid hormone secretion is regulated by the serum ionized calcium concentration (negative feedback mechanism) to maintain calcium levels in a normal range (8.8–10.4 mg/dL).

TABLE 46-7 MANAGEMENT OF MYXEDEMA COMA

Tracheal intubation and controlled ventilation of the lungs as needed

Levothyroxine: 200–300 mg IV over 5–10 min

Cortisol: 100 mg IV and then 25 mg IV every 6 hr

Fluid and electrolyte therapy as guided by serum electrolyte measurements

Warm environment to conserve body heat

IV = intravenous.

B. Hyperparathyroidism

1. Hypercalcemia is responsible for a broad spectrum of signs and symptoms (nephrolithiasis, confusion).

2. Treatment and Anesthetic Considerations. Preoperative IV administration of normal saline and furosemide may lower serum calcium concentrations. There is no evidence that a specific anesthetic drug or technique is preferred. A cautious approach to the use of muscle relaxants is suggested by the unpredictable effect of hypercalcemia at the neuromuscular junction. Careful positioning of osteopenic patients during surgery is necessary to minimize the likelihood of pathologic bone fractures.

3. Anesthesia for Parathyroid Surgery. General anesthesia is most commonly selected. Minimally invasive parathyroidectomy is superior to conventional bilateral cervical exploration and can usually be performed using a bilateral cervical plexus block.

C. Hypoparathyroidism. Clinical features are manifestations of hypocalcemia, and treatment is with calcium gluconate (10–20 mL of 10% solution IV) (Table 46-8).

III. ADRENAL CORTEX

A. The biologic effects of adrenal cortex dysfunction reflect cortisol or aldosterone excess or deficiency (Table 46-9).

B. Glucocorticoid Excess (Cushing Syndrome) (Table 46-10)

1. The diagnosis of hyperadrenocorticism is established by failure of the exogenous administration of dexamethasone to suppress endogenous cortisol secretion.

2. Anesthetic Management (Table 46-11). Etomidate has been used for temporizing medical treatment of severe Cushing’s disease because of its inhibition of steroid synthesis.

C. Mineralocorticoid excess should be considered in nonedematous hypertensive patients who have persistent hypokalemia and are not receiving potassium-wasting diuretics.

TABLE 46-8 MANIFESTATIONS OF HYPOCALCEMIA

Neuronal irritability

Skeletal muscle spasms

Congestive heart failure

Prolonged Q-T interval on the electrocardiogram

TABLE 46-9 COMPARATIVE PHARMACOLOGY OF CORTICOSTEROIDS^*

^*The glucocorticoid and mineralocorticoid properties of cortisol are considered to be equivalent to 1.

D. Adrenal Insufficiency (Addison Disease)

1. Clinically, primary adrenal insufficiency is usually not apparent until at least 90% of the adrenal cortex has been destroyed.

2. The clinical presentation almost always includes hypotension. (A high degree of suspicion should be maintained for patients who demonstrate cardiovascular instability without a defined cause.)

3. Treatment and Anesthetic Considerations. Immediate therapy consists of electrolyte resuscitation (glucose in normal saline) and steroid replacement (100 mg IV every 6 hours for 24 hours). Inotropic support is indicated if hemodynamic instability persists despite adequate fluid resuscitation.

TABLE 46-10 MANIFESTATIONS OF GLUCOCORTICOID EXCESS

Truncal obesity and thin extremities (reflects redistribution of fat and skeletal muscle wasting)

Osteopenia

Hyperglycemia

Hypertension: (fluid retention)

Emotional changes

Susceptibility to infection

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