Thyroidectomy

Etiology

Thyroid hormones are T4 and T3

• T4 - Thyroxine - half-life of 6 days
• T3 - Triiodothyronine - half-life of one day
  • rT3 is an inactive form of T3

T4 is the most prevalent, but T3 is the most potent and active form—ratio 4:1

Deiodinase enzyme converts T4 to T3 (from the liver/kidney and other tissues)

Thyroid hormones are synthesized in the follicular epithelial cells, and the follicular lumen stores newly synthesized thyroid hormones attached to thyroglobulin

TRH is released from the hypothalamus, stimulates the release of TSH from the anterior pituitary, and stimulates the thyroid gland to release T4 and T3—negative feedback to the anterior pituitary

Patients presenting for thyroid surgery are usually made euthyroid before elective surgery and may be taking

• Propylthiouracil (thyroid hormone antagonist, blocks synthesis of T3, T4 peripherally)
• Methimazole (thyroid hormone antagonist - inhibits synthesis of thyroid hormones by decreasing iodine use in the manufacture of thyroglobulin and iodothyronine)
• Potassium iodide (thyroid hormone antagonist, fosters colloid accumulation in thyroid follicles, decreases vascularity of gland)
• Glucocorticoids or Beta-blockers

Effects of Thyroid Hormones

Growth and maturation

• Skeletal muscle and CNS, required for normal mentation in adults (Cretinism)

Autonomic nervous system

• Enhances effects of catecholamines
• Increases HR
• Lipolysis
• Gluconeogenesis
  • Generation of glucose from non-carbohydrate carbon substrates such as lactate, glycerol, and glucogenic amino acids

Basal metabolic rate (BMR)

• Oxidative metabolism, increased ATP production
• Thyroid hormone regulates basal metabolic rate
• Increased oxygen consumption - oxygen in phosphorylation (addition of a phosphate (PO4) group to a protein or other organic molecules) leading to an increase in heat

Carbohydrate, lipid, and protein metabolism

• Stimulates glycogenolysis and gluconeogenesis, lipogenesis and lipolysis, protein catabolism

> 99% of the hormone is bound to plasma proteins, providing a circulating reservoir

Preoperative Diagnosis

Definite/suspicious/inconclusive findings for malignancy, goiter, thyroid cancer (papillary, follicular, medullary, anaplastic (rare, 1%)), thyroid nodule, hyperthyroidism, Grave's disease

Hyperthyroidism

Elevated T3, T4, decreased TSH

Secondary to Graves' disease, toxic multinodular goiter, thyroid adenomas, TSH-secreting tumor (rare), or overdose of thyroid hormone

Symptoms

Fatigue, sweating, intolerance to heat

Increased appetite, weight loss, or gain

A-fib, palpitations are usually the first symptoms, CHF

Increased BP, HR, and pulse pressure

Increased thyroid levels

Thyroid goiter

Exophthalmos (antibodies to eye orbital muscles are common)

Nervousness, agitation

Thyroid Storm

Life-threatening exacerbation of hyperthyroidism during periods of stress. It can be mistaken intraoperatively for malignant hyperthermia, sepsis, anaphylaxis, and other hypermetabolic reactions

Symptoms

Hyperthermia >40 degrees C

Tachycardia

Widened pulse pressure

Anxiety

Altered mental status leading to psychosis, coma, and myopathy

Treatment

Increase FIO₂

Fluid resuscitation

Electrolyte replacement/correction (increased Ca++)

Cooling blankets

Acetaminophen

Maintain diuresis

Treat precipitating events (infection, CHF, DKA, pregnancy)

Specific Treatment

Propylthiouracil (blocks synthesis)

• Block synthesis BEFORE giving iodides to block release
  • Otherwise, "iodine escape" will occur later, and thyroid hormones will be produced
  • 1 hr is adequate

Sodium Iodide (blocks release)

Steroids (unclear mechanism)

Beta-blockers (caution reactive airway, CHF)

Anesthetic Consideration

Ensure euthyroid state (check temp, HR, pulse pressure, and reflexes)

Supine position with extended neck and pillow under upper back to expose surgical site

Tracheal compression from large goiters can cause tracheal deviation, stridor

Increased basic metabolic rate causes increased Vo2 (oxygen uptake) and rapid desaturation on induction

Beta-blocker therapy for increased HR

Generous sedation due to nervousness

Avoid agents that stimulate the sympathetic nervous system

• Ketamine, pancuronium, meperidine

Visualize vocal cords function before extubation

The ability to phonate "e" post-extubation implies continued vocal cord function

Avoid coughing on emergence

Maintain normothermia

Emergence

There is an increased risk of airway obstruction after this procedure

• Secondary to recurrent laryngeal nerve damage
• Bilateral: the patient is unable to speak and requires reintubation
• Unilateral: hoarseness

Tracheomalacia (weakness and floppiness of tracheal cartilage) or hematoma

• Rapid intubation may be lifesaving if hematoma reopens the incision and drains blood

Acute hypoparathyroidism (hypocalcemia)

• It can present as a laryngeal stridor 24-48 hrs postop and may start with tingling of fingertips and lips, which, untreated, can progress to tetany and seizures
• Give 1 amp of Ca⁺⁺ gluconate iv over 20 min will usually alleviate symptoms

Hypothyroidism

Decreased T3, T4, elevated TSH

Autoimmune disease destroys follicular epithelial cells. The patient cannot make thyroid hormone

Seen in elderly patients

Symptoms

Cold intolerance

• Depression of BMR

Decreased appetite and weight gain due to depression of BMR with low protein and fat metabolism

Decreased HR, pulse pressure

Edema (Myxedema)

• Accumulation of protein - mucopolysaccharides under the skin, water in interstitial space

Cretinism

• CNS development impaired
• Replace thyroid deficiency in the first few days to prevent long-term developmental side effects

Myxedema Coma

Severe hypothyroidism has mortality >50%

Symptoms

Stupor or coma

Hypothermia

Hypoventilation with hypoxemia

Bradycardia HR 50-60

Hypotension

Apathy

Hoarseness

Hyponatremia

Treatment

Early intubation

Treat low BP cautiously with volume, inotropes, pacing

Passive warming if temp >30C

Specific Treatment

L-Thyroxine (T4) or

Triiodothyronine (T3)

Hydrocortisone

Deceased TSH earliest sign of response to treatment

Anesthetic Considerations

Ensure euthyroid state

• Check temp, HR, pulse pressure, and reflexes

Supine position with extended neck and pillow under upper back to expose surgical site

Tracheal compression from large goiters can cause tracheal deviation, stridor

May see increased sensitivity to anesthetic agents and muscle relaxants

Decreased ventilatory response to increased CO₂ and reduced O₂

Bradydysrhythmias, diastolic HTN, decreased LV compliance, pericardial effusions

Addison's disease occurs in 5-10% - (no aldosterone, no cortisol) - may need steroids during surgery due to stress

Decreased basic metabolic rate, temperature

May have joint pain, myalgia

Anemia, GI bleeding, constipation, ileus

Visualize vocal cords function before extubation

The ability to phonate "e" post-extubation implies continued vocal cord function

Avoid coughing on emergence

Maintain normothermia

Emergence

There is an increased risk of airway obstruction after this procedure

• Secondary to recurrent laryngeal nerve damage
• Bilateral: the patient is unable to speak and requires reintubation
• Unilateral: hoarseness

Tracheomalacia (weakness and floppiness of tracheal cartilage) or hematoma

• Rapid intubation may be lifesaving if hematoma reopens the incision and drains blood

Acute hypoparathyroidism (hypocalcemia)

• It can present as a laryngeal stridor 24-48 hrs postop and may start with tingling of fingertips and lips, which, untreated, can progress to tetany and seizures
• Give 1 amp of Ca⁺⁺ gluconate iv over 20 min will usually alleviate symptoms