THYROID DISORDERS AND ANAESTHESIA

PRESENTOR :DR. RAJESH CHOUDHURI

MODERATOR: DR. C.R. MONDAL, PROF. & HOD PGT, DEPARTMENT OF ANAESTHESIOLOGY

AGMC & GBP HOSPITAL, AGARTALA

THYROID GLAND• Thyroid Gland is H-shaped ,Right and left lobe with isthmus.• Location of Thyroid Gland: Anterior to trachea.

Just below cricoid cartilage. Covering second through fourth tracheal rings Thyroid gland weighs about 20 gm.

• 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.

• 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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THYROID GLAND• Histological structure: • composed of numerous follicles filled

proteinaceous colloid. Also contains parafollicular C

cells, which produce calcitonin.

•

• Regulation of thyroid secretion

THYROID HORMONE SYNTHESIS

THYROID HORMONE: PHYSIOLOGY -T4/T3 ratio in blood is 10:1.

- -In blood, T4 and T3 bind reversibly to three major proteins: TBG (80%), prealbumin(10%) and albumin ( 5% to 10%). -T3 is 3-4 times more active than T4.

SITE OF ACTION: Cell nucleus → stimulates m RNA synthesis → controles protein synthesis. Mitochondria→ oxidative phosphorylation and ATP formation.

Plasma membrane→influences transcellular flux of substrate and cations.

FUNCTIONS: 1. stimulates all metabolic processes. 2. influences growth and maturation of tissues,

enhance tissue function. 3. stimulates protein synthesis ; carbohydrate and lipid metabolism. 4. Cardiac: acts directly on cardiac myocytes and vascular smooth muscle cells. Increases myocardial contractility, decreases SVR, increases intravascular volume, increases number of beta adrenergic rceptor. 5. CNS: effect on neuronal function and reflexes. Reaction time of stretch reflex is shortened in hyperthyroidism. Also affects RAS.

SICK EUTHYROID SYNDROME Abnormal thyroid function tests that occur in the setting of acute and

severe nonthyroidal illness without pre-existing hypothalamic-pituitary and thyroid gland dysfunction.

Most common findings are a low T3,T4 and TSH. Reversible after recovery from the illness. Partly caused by cytokines or other inflammatory mediators acting at

the hypothalamus, pituitary, thyoid gland and hepatic deiodinase system.

Degree of abnormality correlates with the disease severity. Administration of thyroid hormones in this situation is controversial

and has not been shown to improve outcomes.

HYPERTHYROIDISM• CAUSES: 1. Graves disease—most common cause.

2. toxic multinodular goiter. 3. TSH secreting pituitary tumor.

4. functioning thyroid adenomas. 5. overdose of thyroid replacement medications.

6. S/E of amiodarone/ irradiation thyroiditis.

• DIAGNOSIS: made by abnormal TFTs, elevated total and free T4, T3, low TSH, raised free thyroxine index.

A TSH level of 0.1-0.4 munits/L with normal level of FT3 and FT4 is diagnostic of subclinical hyperthyroidism.

A TSH level of less than 0.03 munits/L with elevated T3 and T4 is diagnostic of overt hyperthyroidism.

HYPERTHYROIDISM

• CLINICAL MANIFESTATION:classical symptoms: hyperactivity,

weight loss and tremor. Other symptoms: palpitation, anxiety/nervousness, diarrhea, intolerance to heat, large muscle group weakness, menstrual abnormalities. Signs: tachycardia ( ↑ sleeping PR), warm moist skin, irregularly irregular pulse, fine brittle hair, ↑ CO, IHD, HF . Eye signs: 1. Eyelid retraction.

2. Lid lag sign. 3. Joffroy

sign-absence of wrinkling. 4. Mobius sign-difficulty in convergence.

5. Stellwag’s sign-absence of blinking.

HYPERTHYROIDISMTREATMENT:

1. Antithyroid drugs: methimazole or proylthiouracil ( PTU) -interfere with thyroid hormone synthesis. PTU also inhibits the peripheral conversion of T4 to T3.

- euthyroid state can almost always be achieved within 6-8 weeks. - S/E: agranulocytosis, hepatotoxicity, vasculitis, teratogenicity.

2. Iodide: Inhibit hormone release. Effects occur immediately but short –lived. Reserved for hyperthyroid patients for surgery, thyroid storm, severe thyrocardiac disease.

Potassium iodide- 3 drops PO every 8 hrly for 10-14 days.Lithium carbonate 300 mg PO every 6

hrly . 3. beta adrenergic antagonists: relieve signs and symptoms of increased adrenergic activity. Propanolol has the added feature-inhibit conversion of T4 to T3. 4. radioactive iodine and subtotal thyroidectomy: other alternative to medical therapy.

HYPERTHYROIDISM: ANAESTHETIC CONSIDERATIONS

• Pre-operative considerationAntithyroid medications and beta blockers should be continued through

the morning of surgery.Miller: ideally patients should be rendered euthyroid prior to any elective

procedure . Begining pre-op antithyroid medication take 2-6 weeks for effect, can use KI with beta-blockers in addition or alternatively.

Benzodiazepines are good choice for pre-medication.Carefull evaluation of air-way.

HYPERTHYROIDISM: ANAESTHETIC CONSIDERATIONS

• Intra-operative considerations:No controlled study suggest advantages of particular anaesthetic

drug or technique for hyperthyroid patients, however:Drugs that stimulate SNS should be avoided because of the

possibility of large increase in BP and HR. Ex-ketamin, pancuronium, atropine, ephedrine.

Thiopental may be the induction agent of choice as it possesses antithyroid activity at high doses.

Close monitoring of cardiac function and body temperature. Need for invasive monitoring?

Adequate anaesthetic depth should be obtained prior to laryngoscopy or surgical stimulation to avoid tachycardia, Htn., ventricular arrhythmia.

HYPERTHYROIDISM: ANAESTHETIC CONSIDERATIONS• Intra-operative considerations:Anticipate exacerbated hypotensive response during induction as

patient may be hypovolaemic.Eye protection.Muscle relaxants can be used safely. Note: patients with autonomic

thyrotoxicosis are associated with increased risk of myopathies and myesthenia gravis.

Reversal with glycopyrolate instead of atropine.Hyperthyroidism doesn’t increase MAC requirements, volatile agents can

be used safely.

HYPERTHYROIDISM: ANAESTHETIC CONSIDERATIONS• Post-operative considerations: thyroid storm is the most-serious post operative problem.Precipitating factors: trauma, infection, medical illness or surgery.Characterized by: hyperpyrexia, tachycardia, hypermetabolism, altered

conciousness and hypertension.Incidence is 10% in patients hospitalized for thyrotoxicosis.Onset is 6-24 hrs after surgery, but can happen intra-operatively mimicking MH.Thyroid hormone levels may not be significantly higher than during

uncomplicated hyperthyroidism.Unlike MH, not associated with muscle rigidity, ↑ CPK or marked degree of lactic or

respiratory acidosis.

HYPERTHYROIDISM: ANAESTHETIC CONSIDERATIONSThyroid storm: treatment IV hydration with glucose containing crystalloids and cooling

measures.Beta-blockers: IV propanolol ( 0.5 mg increments) , esmolol to control

HR until < 90/m.PTU 200-400 microgram every 8 hrly orally or by NG tube/rectally.Sodium iodide 1 gm over 12 hrs.Correction of any precipitating events ( infection).Dexamethasone 2 mg every 6 hrly or Cortisol 100-200 mg every 8 hrly.Mortality rate is approximately 20%.

ANAESTHETIC CONSIDERATIONS: SUBTOTAL THYROIDECTOMY

• Associated with several complications: recurrent laryngeal nerve palsy can cause hoarseness if

unilateral , or stridor if bilateral.Vocal cord function may be evaluated by DL after deep

extubation if there is concern.Haematoma formation may cause airway compromise . May

require immediate opening of neck wound.Hypothyroidism may result from unintentional removal of

parathyroid gland . Hypocalcaemia will result within 24-72 hrs.Pneumothorax-may be developed.

HYPOTHYROIDISM• INCIDENCE: 0.5% TO 0.8% of adult population; ten times more

common in females.• CAUSES: - primary hypothyroidism—95% of all cases.

-autoimmunue ( Hashimoto’s thyroiditis) -post radioactive iodine.

-post thyroidectomy. -overdose of

anti-thyroid medication. - iodine deficiency.

-secondary hypothyroidism( failure of the hypothalamo-pituitary axis)

• DIAGNOSIS: can be confirmed by low free thyroxin levels and elevated TSH( if free). A TSH level of 5.0 to 10 milliunits/L with normal levels of FT3 and FT4 is diagnostic of subclinical hypothyroidism.

A TSH level of more than 20 milliunits/L with reduced levels of T3 and T4 is diagnostic of overt hypothyroidism.

HYPOTHYROIDISM• CLINICAL MANIFESTATIONS:Hypothyroidism in early neonatal development may result in cretinism.In adults, manifestation can be subtle: weight gain, cold intolerance,

muscle fatigue, lathergy ,constipation, hypoactive muscle reflexes, depression, periorbital or pre-tibial swelling.

HR, contractility , stroke-volume and CO decreases, extremity may be cold, hair may be coarse and brittle, large tongue.

Anaemia, hypoglycaemia, hyponatraemia, ↑ cholesterol levels.

ECG: flattened or inverted T waves, low amplitude P waves and QRS complexes, sinus bradycardia, ventricular dysarrythmia.

HYPOTHYROIDISM• TREATMENT:• Oral replacements.• L-thyroxine: started with 50-100 microgram ( 25 mcg in the elderly or in

the patients with IHD)Titrated by clinical improvement and by

monitoring TSH level. T4 has a half-life of 7 days, onset of action 12 hrs and takes almost 2

weeks for peak action.T3 has a half-life of 1.5 days and is available in injected form.

HYPOTHYROIDISM: ANAESTHETIC CONSIDERATION• PRE-OPERATIVE:Patients with uncorrected severe hypothyroidism ( T4<1 mcg/dl) or

myxedema coma should not undergo elective surgery. Potential for severe cardiovascular instability intra-operatively and myxedema coma.

If emergency surgery is necessary, in patients with overt ds. Or myxedema coma , IV thyroxine and steroid coverage.

Euthyroid state is ideal, however, subclinical cases of hypothyroidism has not been shown to significantly increase risk of surgery.

Continue thyroid replacement medication on morning of surgery.

HYPOTHYROIDISM: ANAESTHETIC CONSIDERATION• PRE-OPERATIVE: Air-way evaluation : patients tend to be obese, large tongue, short

neck, swelling of upper airway.Pre-op sedation should be administered cautiously if at all, as patients

are more prone to drug induced respiratory depression from sedation and narcotics.

Consider aspiration prophylaxis-delayed gastric emptying.Increased incidence of adrenocortical insufficiency and reduced

adrenocorticotropic hormone response to stress—patients should receive hydrocortisone cover during surgery.

Specific investigations: Hb, platelet count and clotting tests, serum electrolytes, Bld. Sugar, ECG.

HYPOTHYROIDISM: ANAESTHETIC CONSIDERATION• INTRA-OPERATIVE:Patients are more sensitive to hypotensive effects of anaesthetic agents

because of decreased CO, blunted baroreceptor reflexes and decreased intravascular vol,; invasive monitoring on a per patient basis.

Ketamin or etomidate may be induction agent of choice.Succinylcholine and NDMRs are generally safe for use; monitor with peripheral

nerve stimulator.Controlled ventilation is recommended as patients tend to hypoventilate.Hypothermia occurs quickly and difficult to prevent and treat.MAC is essentially unchanged.Haematological ( anaemia, platelet, coag dusfx) abnormalities, electrolyte

imbalance and hypoglycaemia are common and require close monitoring intra-operatively.

HYPOTHYROIDISM: ANAESTHETIC CONSIDERATION• MYXEDIMA COMA:Rare form of decompensated hypothyroidism.Characterised by stupor or coma, hypoventilation, hypothermia,

bradycardia, hypotension and severe dilutional hypontraemia( SIADH) , CHF.

Medical emergency with mortality rate of 15-20%.Infection, trauma, cold, CNS depressants predispose hypothyroid

patients, especially in elderly.Treatment: IV thyroxine is indicated( L-thyroxine loading dose 300-500 mcg

followed by 50 mcg/day for 24-48 Hrs) IV hydration with

dextrose containing crystalloids , correction of electrolyte imbalance.

Support cardio-vascular and pulmonary system as necessary.

OTHER PERIOPERATIVE CONSIDERATIONSEyes should be protected especially if exophthalmos is present.The patient is positioned slightly head up to help venous drainage.Neck is hyper extended and should be well-established.Extension tubing for iv lines and long respiratory hoses may be

required.Valsalva maneuver in Trendelenberg position is carried out to check

hemostasis.Steroids may be given if extensive tracheal handling and edema

suspected.Extubation should be smooth and coughing should be avoided to

prevent bleeding.The possibility of tracheomalacia and vocal cord palsy should be kept in

mind.Surgeon may wish to observe the movement of vocal cord at the end of

operation.