Thyroid Surgery Indications and Preparation

Thyroidectomy Indications

• Hyperthyroidism (increased thyroid function)
• Cancer or suspected cancer
• Hypothyroidism (decreased thyroid function) with a goiter that causes respiratory difficulty
• Idiopathic hypertrophy (enlargement of the thyroid gland) without increased thyroid hormone synthesis and release

Preoperative Care

• Patients should be rendered euthyroid before surgery
• Focus on avoiding stimulation of the sympathetic nervous system
• Prevent systemic effects of hyperthyroidism (exaggerated sympathetic nervous system responses)

Perioperative Care

• Maintain hemodynamic stability
• Provide adequate fluid resuscitation
• Administer antithyroid medications preoperatively
• Avoid excessive sympathetic stimulation

Postoperative Complications

• Thyroid storm: a potentially fatal complication that can occur during intraoperative and postoperative periods

Anatomy of the Thyroid Gland

• The thyroid gland consists of two lobes connected by an isthmus.
• The isthmus is located just below the cricoid cartilage, binding the thyroid gland to the anterior and lateral aspects of the trachea.
• Two pairs of parathyroid glands are located on the posterior aspect of each lobe, regulating plasma calcium levels.

Location and Relationships

• The thyroid gland is positioned near the carotid sheath and the esophagus.
• The recurrent laryngeal nerve (RLN) and the external superior laryngeal nerve (SLN) are branches of the vagus nerve, innervating the intrinsic muscles of the larynx.
• The RLN and SLN are in proximity to the thyroid gland and can be damaged during resection.

Blood Supply and Venous Drainage

• Arterial blood is supplied by the superior and inferior thyroid arteries.
• A venous plexus is formed by the superior, middle, and inferior thyroid veins.
• The thyroid gland is adjacent to the common carotid artery and the anterior jugular vein, potential sources of rapid and massive blood loss.

Thyroid Hormone Regulation

• The hypothalamus secretes thyrotropin-releasing hormone (TRH), stimulating the anterior pituitary gland to create TSH.
• TSH controls the biosynthesis of thyroid hormone.
• The thyroid gland produces and secretes thyroid hormones T3 and T4 in a ratio of 1:10, respectively.

Thyroid Hormone Function

• T3 and T4 have an inhibitory effect on the hypothalamus and anterior pituitary gland, creating a negative-feedback loop.
• T3 is approximately four times more potent than T4.
• Thyroid hormones regulate the metabolic rate of numerous physiologic processes, including tissue growth, oxygen consumption, and energy utilization.

Additional Functions

• The thyroid gland produces calcitonin from parafollicular C cells, helping regulate serum calcium levels.
• Thyroid hormones regulate the metabolic rate, influencing the symptomatology associated with thyroid dysfunction.

Hyperthyroidism

• Hyperthyroidism is a pathologic state characterized by excessive secretion of T3 and T4, leading to hypermetabolism.
• Most cases are caused by Graves' disease, toxic multinodular goiter, or toxic adenoma.
• Signs and symptoms include:
  • Weight loss
  • Exophthalmos
  • Heat intolerance
  • Tachycardia
  • Muscle weakness
  • Hyperglycemia
  • Increased deep tendon reflexes
  • Fatigue
• Cardiac manifestations:
  • Tachycardia
  • Dysrhythmias (atrial fibrillation is most common)
  • Increased cardiac output due to adrenergic hyperactivity
• Surgical intervention:
  • Ideally, surgery should be postponed until patient is euthyroid after taking antithyroid medications
  • If not possible, beta-adrenergic antagonists are used to maintain hemodynamic stability

Hypothyroidism

• Hypothyroidism occurs when T3 and T4 levels are low, often caused by autoimmune diseases (e.g., Hashimoto thyroiditis), radioactive iodine, antithyroid medications, or iodine deficiency.
• Clinical presentation reflects a decrease in metabolic rate.
• Decreased cardiac output is caused by a decrease in both heart rate and stroke volume.
• Mild hypothyroidism is not a contraindication to surgery.
• Severe hypothyroidism (myxedema coma) may increase risk for perioperative complications associated with physiologic stress of surgery and anesthesia.

Surgical Procedure for Thyroid Gland Removal

• The surgical procedure involves a transverse neck incision through the platysma and strap muscles to expose the thyroid gland and its blood supply.
• Hemostasis is achieved before resection can begin.
• The type of thyroidectomy performed depends on the pathologic condition, and can be either subtotal or lobectomy (removal of one lobe of the thyroid gland).
• The amount of thyroid gland tissue removed depends on the severity of thyroid disease and intraoperative findings.
• The muscle and fascia layers are closed using dissolvable material.
• The wound edges are approximated and then sutured or stapled.

Preoperative Period for Thyroidectomy

• Elective cases should be postponed until antithyroid medications allow the patient to achieve a euthyroid state.
• Key findings to evaluate preoperatively include signs and symptoms of hyperthyroidism such as tachycardia, fever, dysrhythmias, or agitation.
• Airway assessment and management may be complicated in patients with an enlarged thyroid gland, which can displace the larynx and distort normal airway anatomy.
• A potentially difficult intubation may occur due to the enlarged thyroid gland, and the anesthetist should be aware of this possibility.
• The quality of the patient's voice and any changes should be noted, as hoarseness may indicate RLN palsy caused by compression from the enlarged thyroid gland.
• Patients who experience difficulty breathing in the supine position may have tracheal compression and occlusion due to pressure from the thyroid gland.

Medications for Achieving a Euthyroid State

• Thioamides (propylthiouracil and methimazole) decrease the formation of thyroid hormone by inhibiting thyroid peroxidase and the organification process.
• Methimazole is 10 times more potent than propylthiouracil.
• Iodides (sodium iodide or potassium iodide) inhibit the organification process, production and release of thyroid hormone, and size and vascularity of the thyroid gland if hyperplasia exists.
• The Wolff-Chaikoff effect is an autoregulatory process that inhibits thyroid activity for several days to weeks if excessive quantities of iodide are present.
• Radioactive iodine (isotope I'31) is administered orally, collects within the follicle, and emits radiation that destroys the thyroid cells, allowing patients to achieve a euthyroid state within several weeks.
• Beta-adrenergic blockade (beta-blocking medications) inhibits the peripheral conversion of T4 to T3 and decreases cardiac manifestations consistent with enhanced sympathetic nervous system activity.
• Corticosteroids (steroids) have an inhibitory effect on TSH and can be used in conjunction with other medications to achieve a euthyroid state.

Anesthetic Induction for Thyroidectomy

• Propofol and etomidate are acceptable medications for anesthetic induction in patients undergoing thyroidectomy.
• Ketamine is not recommended due to its potential to stimulate the sympathetic nervous system.

Airway Management

• Consider awake intubation using a flexible intubating endoscope (fiber-optic scope) if the thyroid mass or goiter causes significant displacement of normal laryngeal anatomy.
• Standard induction and intubation using video laryngoscopy (VL) is adequate if there is minimal to no laryngeal displacement.
• A bougie stylet may help with endotracheal tube delivery during VL.

Patient Positioning

• Patients are placed supine with the head elevated 30 degrees and the neck extended.
• The eyes should be carefully taped, especially for patients with exophthalmos, to avoid corneal abrasions.
• There is a possibility of endotracheal tube migration during neck extension and flexion.
• The surgeon may request a shoulder roll and extension of the patient's head to achieve the ideal position for maximal surgical exposure.
• Supporting the occiput is important to avoid postoperative neck discomfort or brachial plexus injury.

Maintenance of Anesthetic Depth

• Maintenance of anesthesia should focus on attenuating sympathetic nervous system stimulation.
• Sevoflurane is an acceptable inhalational anesthetic due to its low blood gas solubility and cardiovascular stability.
• Narcotics and dexmedetomidine can help attenuate the sympathetic nervous system response and provide postoperative analgesia.

Hemodynamic Stability

• Hemodynamic stability should be closely monitored and managed to maintain stability and prevent thyrotoxic crisis.
• The ECG may reveal atrial fibrillation, a common tachydysrhythmia seen with hyperthyroidism.
• The onset of tachycardia unresponsive to fluid replacement may indicate the need for additional beta-adrenergic receptor blockade.
• Hypotension unresponsive to fluid resuscitation is best treated with a direct-acting vasopressor such as phenylephrine.

Nerve Integrity Monitoring (NIM)

• The NIM endotracheal tube uses electromyographic (EMG) information to determine the real-time integrity of the right and left RLNs.
• The electrodes on the proximal end of the tube are inserted into a monitor that interprets the signals and allows the surgeon to determine if RLN function remains intact throughout surgery.
• Neuromuscular blockade and laryngeal tracheal lidocaine should be avoided if the NIM endotracheal tube is used.

Thyroid Storm

• Early detection and prevention of thyrotoxic crisis are essential.
• Signs and symptoms of thyroid storm include hyperthermia, tachycardia, and tachydysrhythmia, central nervous system symptoms, and rhabdomyolysis.
• Treatment of thyroid storm involves pharmacologic treatment to decrease circulating thyroid hormone, as well as providing supportive interventions.
• Treatment options include:
  • Increasing the fraction of inspired oxygen concentration
  • Fluid resuscitation with cooled intravenous fluids
  • Propylthiouracil or methimazole to inhibit thyroid hormone synthesis
  • Sodium iodide to block release of hormone from the thyroid gland
  • Acetaminophen as the antipyretic of choice
  • Beta-adrenergic antagonists to control cardiovascular effects
  • Steroid administration such as dexamethasone or hydrocortisone
  • Serial electrolyte and arterial blood gas analysis

Postoperative Complications of Thyroidectomy

• Recurrent Laryngeal Nerve (RLN) damage can be unilateral or bilateral, causing hoarseness, aphonia, stridor, or respiratory distress.
• Incidence of permanent vocal cord paralysis is rare (0.5% to 2.4%), while temporary paralysis occurs in 2.6% to 5.9% of cases.
• Identification of RLN intraoperatively can prevent RLN damage, but may be difficult in cases of extreme hypertrophy or cancer.
• Vocal cord function can be assessed via direct laryngoscopy after deep extubation or by having the patient phonate "e" postoperatively.

Hematoma Formation

• Hematoma formation can cause compression or collapse of the airway secondary to tracheomalacia, with an incidence of approximately 1%.
• Inadequate surgical hemostasis, coagulopathy, acute hypertension, and straining from postoperative nausea and vomiting can increase the potential for postoperative bleeding.

Treatment of Hematoma Formation

• Definitive treatment involves immediate evacuation of the hematoma and reexploration of the surgical site.

Pneumothorax

• Pneumothorax is a rare complication of thyroidectomy, occurring when air enters the thoracic cavity due to the proximity of the surgical site to the apices of the lungs.

Acute Hypocalcemia

• Acute hypocalcemia can occur due to inadvertent removal of the parathyroid glands, with an incidence of 10% to 50%.
• Serum calcium and parathyroid hormone levels can be checked intraoperatively or immediately postoperatively to predict the potential for hypoparathyroidism.
• Hypocalcemia typically occurs 24 to 48 hours after surgery and can be treated with oral calcium and vitamin D.
• Parathyroid dysfunction is usually transient, but can be permanent if all four parathyroid glands are removed.