Thyroid Gland and Anesthetic Management

1. Thyroid Gland and Anesthetic Management Daniel Stairs CRNA, MSN, MBA Excela Health School of Anesthesia

2. Thyroid Gland is H-shapedRight and left lobe with isthmus

3. Location of Thyroid Gland • Anterior to trachea • Just below cricoid cartilage • Covering second through fourth tracheal rings • Thyroid gland weighs about 20 gm

4. Blood Supply to Thyroid Gland • 4 to 6 cc/min/gm • Arterial supply via inferior and superior arteries • Venous supply via inferior, middle, and superior thyroid veins

5. Nerve Supply • Two superior laryngeal nerves and two recurrent laryngeal nervessupply the entire sensory and motor innervations to the larynx.

6. Innervation

7. Recurrent Laryngeal Nerve • Most common nerve injured in throidectomy • Motor supply • Sensation below vocal cords • With selective injury to abductor fibers: (1) hoarseness (2) bilateral injury (3) obstruction

8. Recurrent Laryngeal Nerve • Selective injury to adduction fibers • Post-operative assessment after thyroidectomy is via laryngoscopy and having patient phonate letter “e” • Most common nerve injury

9. Superior Laryngeal Nerve • Motor supply to cricothyroid muscle (SLN external branch) • Internal branch provides sensation above the vocal cords • Injury causes possible risk for aspiration and hoarseness

10. Essential Thyroid Hormones • Thyroxine – or T4 • Triiodothyronine – or T3 • Release of these hormones into circulation stimulated by TSH • T3 is less firmly bound to carrier proteins and disappears from circulation quicker • T3 is 3-5 times as potent as T4 but is limited by its transient nature

11. Thyroid Hormones • Nearly all circulating T3 is derived from peripheral conversion of T4 • Major Functions of Thyroid Hormones: (1) calorigenic effects (2) growth and cellular differentiation (3) metabolic effects (4) muscular effects

12. Other Functions of Thyroid Hormones • Working with growth hormone, they ensure proper development of the brain • Increase protein breakdown and glucose uptake by cells, enhance glycogenolysis. and depress cholesterol levels • In excess… they may interfere with ATP synthesis and thus speed the exhaustion of energy in muscle tissues

13. Thyroid Hormones • Thyroxine normal serum range is 5-12 mcg/dL • Triiodothyronine normal serum range is 70-90 ng/dL

14. Laboratory Testing of Thyroid Hormone Five General Categories • Direct tests of thyroid function • Tests relating to the concentration and binding of thyroid hormones in blood • Metabolic indexes • Tests of homeostatic control of thyroid function • Miscellaneous tests

15. (1) Direct Tests • In-vivo administration of radioactive iodine • Thyroid Radioactive Iodine Uptake (RAIU) is the most common • RAIU is measured 24 hours after administration of isotope • Normal is 10-30% of administered dose after 24 hours • Values above normal indicate thyroid hyperfunction

16. (2) Tests Related to Hormone Concentration and Binding • Are radioimmunoassays • Highly specific and sensitive radioimmunoassays to measure serum T3 and T4 • Highly sensitive TSH assay is the most sensitive of thyroid function

17. (3) Metabolic Indexes • Although measurement of the metabolic impact of thyroid hormones have value in the investigative setting, none is sufficiently sensitive, specific, and easily performed for routine use • Measurements of oxygen consumption in the BMR were once a mainstay in the diagnosis of thyroid disease, but not today

18. (4) Tests of Homeostatic Control • Basal serum TSH concentration • Thyrotropin-releasing hormone • Thyroid suppression test

19. (5) Miscellaneous Tests • These do not assess thyroid function but are if value in defining the nature of the thyroid disorder or in planning therapy • Example: some patients with autoimmune thyroid disease develop circulating antibodies against T3 and T4 resulting in sporadic highs and lows in the concentration of the hormones

20. Hyperthyroidism • Clinical symptomsinclude: nervousness, palpitations, intolerance to heat, weight loss, muscle weakness, and fatigue • Physical exam: smooth, moist skin,exopthalmus, presence of goiter, tachycardia, and hyperactive tendon reflex. Skin temperature is elevated, and there is fine tremor of the extended hands or a course tremor and jerking of trunk.

21. Hyperthyroidism • Long-standing thyrotoxicosis • Mild anemia and lymphocytosis are common • Approximately 20% will have reduction in total WBC count

22. Hyperthyroidism • Affects approximately 2% of women and 0.2% of men

23. Causes of Hyperthyroidism • Graves’ disease (diffuse goiter and opthalmopathy) is the most common • Graves’ disease typically occurs in women 20 to 40 years of age • An autoimmune pathogenesis for Graves’ disease is suggested by presence of immunoglobulin G autoantiobodies

24. Causes of Hyperthyroidism • Iatrogenic…second most common cause. May result from administration of T3/T4 • Toxic nodular goiter …nodules functioning independently of normal feedback regulation • Thyroiditis …inflammation-induced release of thyroid hormones

25. Treatment of Hyperthyroidism • Antithyroid Drugs • Usual initial medical management • Propylthiouracil,carbimazole, methimazole • These drugs inhibit synthesis of inorganic iodide and coupling of iodothyronines • Graves’ disease often initially treated with antithyroid drugs in hope of inducing a remission or achieving euthyroidism before surgery

26. Treatment of Hyperthyroidism • Pregnant females should be treated with propylthiouracil (of antithyroid drugs it crosses placenta least), minimizing the risk of goiter any hypothyroidism in fetus • Serious side effects of antithyroid drugs include agranulocytosis • Intraoperative bleeding, from drug-induced thrombocytopenia or hypoprothrombinemia has been reported in patients on propylthiouracil • Hypothyroidism is a risk of antithyroid drugs so patient may receive supplemental T4

27. Treatment of Hyperthyroidism Beta-Adrenergic Antagonists • useful adjunctive therapies for patients with Graves’ disease diminish some of the S/S (tachycardia, anxiety, tremor) more rapidly than can antithyroid drugs • Nadolol and atenolol have a longer duration than propranolol • These drugs do not block the synthesis and secretion of thyroid hormones

28. Treatment of Hyperthyroidism Inorganic Iodine • Iodine in pharmacologic doses (Lugol’s solution, 5% iodine, 10% potassium iodide in water) inhibits the release of T3 and T4 for a limited time (days to weeks) after which its antithyroid activity is lost • Inorganic iodine is principally used to prepare pts. for surgery and treat thyrotoxic crisis

29. Treatment of Hyperthyroidism Radioiodine Therapy • Often selected as tx of choice for hyperthyroidism that recurs following therapy with antithyroid drugs • Objective is to destroy sufficient thyroid tissue to cure hyperthyroidism • Permanent hypothyroidism is the only important complication of this therapy • Pregnancy is an absolute contraindication as it may cause ablation of the fetal thyroid gland

30. Treatment of Hyperthyroidism Subtotal Thyroidectomy • Used to treat Graves’ disease when radioiodine is refused, or for rare pts. With large goiters causing tracheal compression or cosmetic concerns • If elective, pt. needs to be rendered euthyroid with drugs • In emergency, pts. can be prepared for surgery in less than 1 hour by IV administration of esmolol

31. Treatments to Render Hyperthyroid Pts. Euthyroid Prior to Surgery • Emergency Surgery Esmolol 100-300 mcg/kg/min IV until heart rate <100/min • Elective Surgery Oral administration of Beta-adrenergic antagonist (propranolol, nadolol, atenolol) until heart rate <100/min Antithyroid drugs Antithyroid drugs plus potassium iodide Potassium iodide plus Beta-adrenergic antagonist

32. Subtotal Thyroidectomy • Some uncommon complications include damage to recurrent laryngeal nerves, postop bleeding into the neck with resultant tracheal compression, and hypoparathyroidism • Most common nerve injury is damage to abductor fibers of recurrent laryngeal • This injury when unilateral…hoarseness, and paralyzed vocal cord assuming an intermediate position

33. Subtotal Thyroidectomy • Bilateral recurrent nerve injury results in aphonia and paralyzed vocal cords • The cords can collapse together, producing total airway obstruction during inspiration • Selective injury of adductor fibers of recurrent laryngeal nerves leaves the adductor fibers unopposed and pulmonary aspiration a hazard

34. Subtotal Thyroidectomy • Airway obstructionthat occurs soon after tracheal extubation, despite normal vocal cord function, suggests tracheomalacia • This reflects a weakening of tracheal rings by chronic pressure of a goiter • Airway obstruction postop (PACU) may be due to tracheal compression by a hematoma

35. Subtotal Thyroidectomy • Hypoparathyroidism resulting from accidental removal of parathyroid gland rarely occurs after subtotal thyroidectomy • If damage to parathyroids does occur, hypocalcemia typically develops 24 to 72 hours postop, but may manifest as early as 1-3 hours postop • Laryngeal muscles sensitive to hypocalcemia…may go from inspiratory stridor progressing to laryngospasm. Prompt IV calcium till laryngeal stridor ceases is tx.

36. Subtotal Thyroidectomy

37. Thyroid Storm (Thyrotoxic Crisis) • Medical Emergency characterized by abrupt appearance of clinical signs of hyperthyroidism (tachycardia, hyperthermia, agitation, skeletal muscle weakness, CHF, dehydration, shock) due to theabrupt release of T4 and T3into the circulation • Can occur intraop but is more likely to occur 16-18 hours postoperative

38. Thyroid Storm (Thyrotoxic Crisis) • When thyroid storm occurs intraop it may mimic malignant hyperthermia • Treatment includes cooled crytalloids and continuous IV infusion of esmolol to maintain heart rate at acceptable level (usually < 100/min) • When hypotension is persistent, the administration of cortisol, 100-200 mg IV may be a consideration

39. Thyroid Storm (Thyrotoxic Crisis) • Propylthiouracil is given in dose of 100mg every 6 hours po or by NG tube to take advantage of the drug’s ability to inhibit extrathyroidal conversion of T4 to T3 • Potassium Iodide is also administered to block the release of T4 to T3 • Also important to treat any suspected infection in these patients

40. Management of Anesthesia • Elective surgery should be deferred until the patient has been rendered euthyroid and the hyperdynamic cardiovascular system has been controlled with Beta adrenergic antagonists, as evidenced by an acceptable heart rate

41. Management of Anesthesia • When surgery cannot be delayed in symptomatic hyperthyroid patients, the continuous infusion of Esmolol, 100 to300 mcg/kg/min IV may be useful for controlling cardiovascular responses evoked by the sympathetic nervous system

42. Management of Anesthesia • Preoperative Medication: (a) benzodiazepines (b) use of anticholinergics not recommended as these drugs could interfere with the body’s own heat-regulating mechanisms and contribute to an increased heart rate

43. Management of Anesthesia • Preoperative: Evaluation of the upper airway for evidence of obstruction (goiter compressing on trachea) is extremely important Be prepared and have available in the O.R. needed equipment for a difficult airway and difficult intubation

44. Management of Anesthesia • Induction: Propoful/Pentothal for induction Ketamine is not a likely selection as it can stimulate the sympathetic nervous system leading to a tachycardia Succinylcholine or non-depolarizers that do not affect the cardiovascular system for intubation (would avoid pancuronium)

45. Maintenance of Anesthesia • Goals in maintenance of anesthesia in patients with hyperthyroidism are: • Avoid administration of drugs that stimulate that stimulate the sympathetic nervous system • Provide sufficient anesthetic-induced sympathetic nervous system depression to prevent exaggerated responses to surgical stimulation

46. Maintenance of Anesthesia • Volatile anesthetics: • isoflurane, desflurane, sevoflurane, are good as they offset adverse sympathetic nervous system responses to surgical stimulation, but do not sensitize the heart to catecholamines • Remember sevoflurane and potential concern with nephrotoxicity caused by an increased production of fluoride owing to accelerated metabolism of this anesthetic

47. Maintenance of Anesthesia • Monitor and keep track of patient’s bodytemperature (keep in mind thyroid storm) • Vigilant monitoring of vital signs • Pts. With exopthalmos prone to corneal ulcerations • For antagonism of neuromuscular blockade with anticholinergics, it is best to avoid atropine and use glycopyrrolate as it has fewer chronotropic effects

48. Maintenance of Anesthesia • Treatment of Hypotension: • When using sympathomimetic drugs must consider the possibility of exaggerated responsiveness of hyperthyroid pts. to endogenous or exogenous catecholamines • Therefore, decreased doses of direct-acting vasopressors such as phenylephrine may be a better choice than ephedrine, which acts in part by provoking the release of catecholamines

49. Regional Anesthesia for Hyperthyroid Patients • Causes a sympathetic nervous system blockade • May be a useful choice in hyperthyroid patients, assuming there is no evidence of high-output congestive heart failure • Continuous epidural may be preferable to spinal because of the slower onset of sympathetic nervous system blockade

50. Regional Anesthesia for Hyperthyroid Patients • If hypotension occurs, decreased doses of phenylephrine are recommended • Epinephrine should not be added to local anesthetics, as systemic absorption of this catecholamine could produce exaggerated circulatory responses