6.3 Hypothyroidism: 

Under secretion of thyroid hormones. Those associated with goiter include: Hashimoto’s thyroiditis is an autoimmune disease in which the thyroid gland is attacked by a variety of cell and antibody mediated immune response. Enlargement of thyroid is due to lymphatic infiltration and fibrosis rather than tissue hypertrophy. The gland enlargement will later lead to thyroid atrophy due to autoantibody destruction of the follicles. 

• Iodine deficiency: A lack of iodine leads to thyroid hormone depletion, Thyroid Stimulating Hormone (TSH) stimulation and gland hypertrophy. 
• Malignancy: The term "malignancy" refers to cancerous cells that have the ability to spread to other sites in the body (metastasize) or to invade and destroy tissues. Malignant cells tend to have fast, uncontrolled growth due to changes in their genetic makeup. Thyroid malignancy will most commonly present as thyroid nodules and are usually minimally active hormonally (patient is euthyroid). 

6.3.1 Manifestation of hypothyroidism: 

Hypothyroidism during neonatal development results in cretinism, a condition marked by physical and mental retardation. Clinical manifestations in the adult are from a generalized reduction in metabolic activity, resulting in lethargy, slow mental functioning, cold intolerance, and slow movements. These patients exhibit bradycardia, decreased cardiac output, and increased peripheral resistance. The diagnosis of hypothyroidism may be confirmed by a low free T4 level. Primary hypothyroidism is differentiated from secondary disease by an elevation in TSH. Ventilatory responsiveness to hypoxia and hypercapnia is depressed in hypothyroid patients. This depression is potentiated by sedatives, opioids, and general anesthesia. Postoperative ventilatory failure requiring prolonged ventilation is rarely seen in hypothyroid patients in the absence of coexisting lung disease, obesity, or myxedema coma. Other abnormalities found in hypothyroidism include anemia, coagulopathy, hypothermia, sleep apnea, and impaired renal free water clearance with hyponatremia. Decreased gastrointestinal motility can compound the effect of postoperative ileus. In long-standing or severe disease, the stress response may be blunted and adrenal depression may occur. 

Treatment of hypothyroidism: The treatment of hypothyroidism consists of oral replacement therapy with a thyroid hormone preparation, which takes several days to produce a physiological effect and several weeks to evoke clear-cut clinical improvement. 

6.3.2 Myxedema coma: 

Myxedema coma represents a severe form of hypothyroidism characterized by stupor or coma, hypoventilation, hypothermia, hypotension, and hyponatremia. This is a medical emergency, requires aggressive therapy. Only life saving surgery should proceed in the face of myxedema coma. Intravenous thyroid replacement (if available) is initiated as soon as the clinical diagnosis is made. Improvements in heart rate, blood pressure, and body temperature may occur within 24 hours. However, replacement therapy with either form of thyroid hormone may precipitate myocardial ischemia. There is also an increased likelihood of acute primary adrenal insufficiency in these patients, and they should receive stress doses of hydrocortisone. Steroid replacement continues until normal adrenal function can be confirmed.

Management of myxedema coma 

• Tracheal intubation and controlled ventilation as needed 
• Levothyroxine, 200–300 μg IV over 5–10 min initially, and 100 μg IV q24h 
• Hydrocortisone, 100 mg IV, then 25 mg IV q6h 
• Fluid and electrolyte therapy as indicated by serum electrolytes 
• Cover to conserve body heat; no warming blankets 

6.3.3 Anesthetic Considerations 

6.3.3.1 Preoperative anesthetic management 

• Patients with uncorrected severe hypothyroidism (T4 < 1 mg/dL) or myxedema coma should not undergo elective surgery and should be treated with thyroid hormone prior to emergency surgery. 
• Thyroid replacement therapy is, however, indicated for patients with severe hypothyroidism or myxedema coma and for pregnant patients who are hypothyroid 
• Although a euthyroid state is ideal, mild to moderate hypothyroidism does not appear to be an absolute contraindication to surgery. 
• Hypothyroid patients usually do not require much preoperative sedation and are very prone to drug induced respiratory depression. In addition, they fail to respond to hypoxia with increased minute ventilation. 
• Consideration should be given to premedicating these patients with histamine H2 antagonists (ranitidine) and metoclopramide because of their decreased gastric-emptying times. 
• Patients who have been rendered euthyroid may receive their usual dose of thyroid medication on the morning of surgery. 

6.3.3.2 Intraoperative anesthetic management: 

• Hypothyroid patients are more susceptible to the hypotensive effect of anesthetic agents because of their diminished cardiac output, blunted baroreceptor reflexes, and decreased intravascular volume. 
• Ketamine is often recommended for induction of anesthesia. The possibility of coexistent primary adrenal insufficiency or congestive heart failure should be considered in cases of refractory hypotension. 
• The maintenance of anesthesia may be safely achieved with titrated either intravenous or inhaled anesthetics. Advised to decrease the minimum alveolar concentration for volatile agents. Regional anesthesia is a good choice in the hypothyroid patient, provided the intravascular volume is well maintained. 
• Monitoring is directed toward the early recognition of hypotension, congestive heart failure, and hypothermia. Scrupulous attention should be paid to maintaining normal body temperature. 
• Decreased cardiac output may speed the rate of induction with an inhalation anesthetic, but hypothyroidism does not significantly decrease minimum alveolar concentration. 
• Potential problems include hypoglycemia, anemia, hyponatremia, difficulty during intubation because of a large tongue, and hypothermia from a low basal metabolic rate. 

6.3.3.3 Postoperative: Recovery from general anesthesia may be delayed in hypothyroid patients by hypothermia, respiratory depression, or slowed drug biotransformation. These patients often require prolonged mechanical ventilation. Patients should remain intubated until awake and normothermic. Because hypothyroidism increases vulnerability to respiratory depression, a nonopioid such as ketorolac would be a good choice for relief of postoperative pain.