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Thyroid Gland and Anesthetic Management

Daniel Stairs CRNA, MSN, MBA

Excela Health School of Anesthesia

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Thyroid Gland is H-shapedRight and left lobe with isthmus

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Location of Thyroid Gland

Anterior to tracheaJust below cricoid cartilageCovering second through fourth tracheal

ringsThyroid gland weighs about 20 gm

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Blood Supply to Thyroid Gland

4 to 6 cc/min/gmArterial supplyArterial supply via inferior and superior

arteriesVenous supplyVenous supply via inferior, middle, and

superior thyroid veins

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Nerve Supply

Two superior laryngeal nerves and two recurrent laryngeal nerves supply the entire sensory and motor innervations to the larynx.

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Innervation

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Recurrent Laryngeal Nerve

Most common nerve injured in throidectomy

Motor supply Sensation below vocal cordsWith selective injury to abductor fibersabductor fibers: (1) hoarseness (2) bilateral injury (3) obstruction

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Recurrent Laryngeal Nerve

Selective injury to adduction fibersadduction fibers Post-operative assessment after

thyroidectomy is via laryngoscopy and having patient phonate letter “e”

Most common nerve injury

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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

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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

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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

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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

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Thyroid Hormones

Thyroxine

normal serum range is 5-12 mcg/dLTriiodothyronine

normal serum range is 70-90 ng/dL

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Laboratory Testing of Thyroid Hormone

Five General Categories(1) Direct tests of thyroid function(2) Tests relating to the concentration and

binding of thyroid hormones in blood(3) Metabolic indexes(4) Tests of homeostatic control of thyroid

function(5) Miscellaneous tests

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(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

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(2) Tests Related to Hormone Concentration and Binding

Are radioimmunoassaysradioimmunoassaysHighly specific and sensitive

radioimmunoassays to measure serum T3 and T4

Highly sensitive TSH assay is the most sensitive of thyroid function

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(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

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(4) Tests of Homeostatic Control

Basal serum TSH concentrationThyrotropin-releasing hormoneThyroid suppression test

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(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

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Hyperthyroidism

Clinical symptomsClinical symptoms includeinclude: nervousness, palpitations, intolerance to heat, weight loss, muscle weakness, and fatigue

Physical examPhysical 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.

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Hyperthyroidism

Long-standing thyrotoxicosis Mild anemia and lymphocytosis are

commonApproximately 20% will have reduction in

total WBC count

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Hyperthyroidism

Affects approximately 2% of women and 0.2% of men

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Causes of Hyperthyroidism

Graves’ diseaseGraves’ disease (diffuse goiter and opthalmopathy) is the most commonmost 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

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Causes of Hyperthyroidism

IatrogenicIatrogenic…second most common cause. May result from administration of T3/T4

Toxic nodular goiterToxic nodular goiter …nodules functioning independently of normal feedback regulation

ThyroiditisThyroiditis …inflammation-induced release of thyroid hormones

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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

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Treatment of Hyperthyroidism

Pregnant females should be treated with propylthiouracilpropylthiouracil (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 propylthiouracilpropylthiouracil

Hypothyroidism is a risk of antithyroid drugs so patient may receive supplemental T4

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Treatment of Hyperthyroidism

Beta-Adrenergic Antagonistsuseful adjunctive therapies for patients

with Graves’ disease diminish some of the S/S (tachycardia, anxiety, tremor) more rapidly than can antithyroid drugs

NadololNadolol and atenololatenolol have a longer duration than propranolol

These drugs do not block the synthesis and secretion of thyroid hormones

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Treatment of Hyperthyroidism

Inorganic IodineIodine 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 Inorganic iodine is principally used to prepare pts. for surgery and treat prepare pts. for surgery and treat thyrotoxic crisisthyrotoxic crisis

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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

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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

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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

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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

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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

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Subtotal Thyroidectomy

Airway obstructionAirway obstruction that 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

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Subtotal Thyroidectomy

HypoparathyroidismHypoparathyroidism resulting from accidental removal of parathyroid gland rarely occurs after subtotal thyroidectomy

If damage to parathyroids does occur, hypocalcemiahypocalcemia 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.

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Subtotal Thyroidectomy

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Thyroid Storm (Thyrotoxic Crisis)

Medical EmergencyMedical Emergency characterized by abrupt appearanceabrupt appearance of clinical signs of hyperthyroidism (tachycardia, hyperthermia, agitation, skeletal muscle weakness, CHF, dehydration, shock) due to the abrupt release of Tabrupt release of T44 and T and T33 into the circulation

Can occur intraop but is more likely to occur 16-18 hours postoperative

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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

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Thyroid Storm (Thyrotoxic Crisis)

PropylthiouracilPropylthiouracil 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 IodidePotassium Iodide is also administered to block the release of T4 to T3

Also important to treat any suspected infection in these patients

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Management of Anesthesia

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

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Management of Anesthesia

When surgery cannot be delayed in symptomatic hyperthyroid patients, the continuous infusion of Esmolol, Esmolol, 100 to100 to 300 mcg/kg/min IV300 mcg/kg/min IV may be useful for controlling cardiovascular responses evoked by the sympathetic nervous system

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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

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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

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Management of Anesthesia

Induction:

Propoful/Pentothal for induction

Ketamine is notis 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)

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Maintenance of Anesthesia

Goals in maintenance of anesthesia in patients with hyperthyroidism are:

(a) Avoid administration of drugs that stimulate that stimulate the sympathetic nervous system

(b) Provide sufficient anesthetic-induced sympathetic nervous system depression to prevent exaggerated responses to surgical stimulation

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Maintenance of Anesthesia

Volatile anesthetics:

(a) isoflurane, desflurane, sevoflurane, are good as they offset adverse sympathetic nervous system responses to surgical stimulation, but do not sensitize the heart to catecholamines

(b) Remember sevoflurane and potential concern with nephrotoxicity caused by an increased production of fluoride owing to accelerated metabolism of this anesthetic

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Maintenance of Anesthesia

Monitor and keep track of patient’s bodyMonitor and keep track of patient’s body temperaturetemperature (keep in mind thyroid storm)

Vigilant monitoring of vital signsPts. With exopthalmos prone to corneal

ulcerationsFor antagonism of neuromuscular

blockade with anticholinergics, it is best to avoid atropine and use glycopyrrolate as it has fewer chronotropic effects

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Maintenance of Anesthesia

Treatment of Hypotension:(a) When using sympathomimetic drugs must

consider the possibility of exaggerated responsiveness of hyperthyroid pts. to endogenous or exogenous catecholamines

(b) Therefore, decreased doses of direct-acting vasopressors such as phenylephrinephenylephrine may be a better choice than ephedrine, which acts in part by provoking the release of catecholamines

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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

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Regional Anesthesia for Hyperthyroid Patients

If hypotension occurs, decreased doses of phenylephrine phenylephrine are recommended

Epinephrine should not be added to local anesthetics, as systemic absorption of this catecholamine could produce exaggerated circulatory responses

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Hypothyroidism

Decreased circulating concentrationDecreased circulating concentration of T3 and T4

Present in 0.5% to 0.8% of adultsDiagnosis based on clinical S/S plus

confirmation of decreased thyroid gland function as demonstrated by appropriate tests

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Hypothyroidism

Causes: The etiology of hypothyroidism is categorized as…

(a)(a) PrimaryPrimary…destruction of the thyroid gland

(b)(b) SecondarySecondary…central nervous system dysfunction

Chronic thyroiditis (Hashimoto’s thyroiditis) Chronic thyroiditis (Hashimoto’s thyroiditis) is the most common causeis the most common cause

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Etiology of Hypothyroidism

Primary Hypothyroidism Thyroid Gland DysfunctionThyroid Gland Dysfunction

Hashimoto’s thyroiditis

Previous subtotal thyroidectomy

Previous radioiodine therapy

Irradiation of the neck

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Etiology of Hypothyroidism

Primary hypothyroidism Thyroid hormone deficiencyThyroid hormone deficiency

Antithyroid drugs

Excess iodide (inhibits release)

Dietary iodine deficiency

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Etiology of Hypothyroidism

Secondary hypothyroidism Hypothalamic dysfunctionHypothalamic dysfunction

Thyrotropin-releasing hormone

deficiency

Anterior pituitary dysfunctionAnterior pituitary dysfunction

Thyrotropin hormone deficiency

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Hypothyroidism

Signs and Symptoms-Decreased metabolic activity-Lethargy is prominent-Intolerance to cold-Cardiovascular changes are often the earliest

clinical manifestations -bradycardia -decreased stroke volume and contractility -decreased cardiac output

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Hypothyroidism

-increased SVR

-systemic hypertension, especially diastolic hypertension occurs in about 15% of hypothyroid patients

-narrow pulse pressure

-increased circulating concentrations of catecholamines

-overt CHF is unlikely, but if present may indicate co-existing heart disease

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Hypothyroidism

Patients with hypothyroidism are predisposed to pericardial effusions

The EKG may reveal low voltage, prolonged PR, QRS, and QT intervals due to pericardial effusion

Conduction abnormalities may predispose patients to ventricular tachycardia, especially torsades de pointes

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Hypothyroidism

Thyroid hormone is necessary for normal production of pulmonary surfactant

Chronic hypothyroidism is associated with pleural effusions

Ventilatory drive to hypoxia and hypercapnia is decreased in patients with severe hypothyroidism

BMR can be decreased up to 50% due to the hypothermia that occurs

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Hypothyroidism

Peripheral vasoconstriction characterized by cool, dry skin

There is often atrophy of the adrenal cortex and associated decreases in the production of cortisol

Inappropriate secretion of ADH can result in hyponatremia owing to the impaired ability of renal tubules to excrete free water

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Hypothyroidism

Treatment -Oral administration of T4

-Pts. With ischemic heart disease and hypothyroidism may not tolerate even modest amounts of T4 without developing angina

-If angina appears or worsens during T4

therapy, coronary angiography and CABG may be necessary before adequate T4 therapy can be achieved

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Myxedema Coma

Rare complication of hypothyroidismManifests as loss of deep tendon

reflexes, spontaneous hypothermia, hypoventilation, cardiovascular collapse, coma, and death

Sepsis in elderly or exposure to cold may be an initiating event

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Myxedema Coma

Treatment is with IV administration of T3, which exerts a physiologic effect within 6 hours

Digitalis, as used to treat CHF, is used sparingly because the hypothyroid patient’s heart cannot easily perform increased myocardial contractile work

Fluid therapy is important, but remember these patients may be vulnerable to water intoxication and hyponatremia

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Hypothyroidism

Management of Anesthesia-Elective surgery should be deferred if

symptomatic-T4 drug has long half-life (7 days) and

administration of it on day of surgery is optional

-T3 drug has shorter half-life (1.5 days) so it may be prudent to have pt. take it on day of surgery

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Hypothyroidism

-Opioid premedication may be exaggerated in the hypothyroid patient

-Supplemental cortisol may be considered if there is concern that surgical stress could unmask decreased adrenal function that may accompany hypothyroidism

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Maintenance of Anesthesia

Induction with pentothal, ketamine, or propoful

Tracheal intubation with succinylcholine, or NDMR, but keep in mind that co-existing skeletal muscle weakness could be associated with an exaggerated drug effect

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Maintenance of Anesthesia

Often achieved with nitrous oxide + short-acting opioids, benzodiazepines, or ketamine

Volatile anesthetics may not be recommended in overtly symptomatic hypothyroid pts. for fear of inducing exaggerated cardiac depression

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Maintenance of Anesthesia

Vasodilation produced by anesthetic drugs in the presence of hypovolemia could result in abrupt decrease in systemic blood pressure

Pancuronium, because of its mild cardiovascular stimulating effects, may be selected for skeletal muscle paralysis

Intermediate and short-acting NDMRs are good as they are less likely to produce a prolonged neuromuscular blockade

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Maintenance of Anesthesia

Monitoring hypothyroid pts. during Monitoring hypothyroid pts. during anesthesia is intended to facilitate anesthesia is intended to facilitate prompt recognition of exaggerated prompt recognition of exaggerated cardiovascular depression, and detection cardiovascular depression, and detection of onset of hypothermiaof onset of hypothermia

Consider arterial line for long surgical procedures, or those associated with significant blood loss

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Maintenance of Anesthesia

IV fluids used should contain sodium to decrease likelihood of hyponatremia

To treat hypotension it is best to use small increments of ephedrine 2.5 to 5.0 mg IV

Phenylephrine could adversely increase SVR in the presence of a heart that cannot reliably increase its contractility

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Maintenance of Anesthesia

Suspect acute adrenal insufficiency when hypotension persistshypotension persists despite treatment with fluids and/or sympathomimetic drugs

Maintain patient’s body temperature with use of a warming blanket or convection system, and warming of IV fluids

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Perioperative Period Possibilities

Increased sensitivity to depressant drugsHypodynamic cardiovascular system

responses…decreased heart rate, decreased cardiac output

Slow metabolism of drugsHypovolemiaDelayed gastric emptyingHyponatremia

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Perioperative Period Possibilities

Impaired ventilatory responses to arterial hypoxemia or hypercarbia

HypothermiaHypoglycemiaAdrenal insufficiency

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Postoperative Management

Recovery from sedative effects of anesthetic drugs may be delayed

Tracheal extubation should be delayed until the hypothyroid patient responds appropriately and their body temperature is near 37 degrees C

Due to increased sensitivity to opioids, may want to consider nonopioid analgesic

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Extreme Goiter

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Goiter

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Shift of Trachea from Enlarged Right Lobe of Thyroid Gland