Tim McCulloch, September 1995

Anaesthetic Implications
Of Thyroid Disease

CLINICAL DISEASE STATES

1. Hyperthyroidism

The commonest cause of hyperthyroidism is Graves' Disease, an auto-immune disease associated with rheumatoid arthritis, pernicious anaemia & myasthenia gravis. Caused by a thyroid-stimulating immunoglobulin which binds to the TSH receptor.

Clinically the patient should have one or more of:

• diffuse goitre and/or hyperthyroidism
• pre-tibial myxoedema
• exopthalmos

Other Causes

• toxic nodular goitre (nodular goitre is usually euthyroid)
• well-differentiated Ca
• pituitary tumor or pit. unresponsive to T3/T4
• other tumors
• thyroxine medication
• transient hyperthyroidism in thyroiditis

A. Manifestations of Hyperthyroidism

Symptoms:

• weight loss
• palpitations, breathlessness
• anxiety, emotional lability, psychosis
• fatigue, weakness
• sweating, heat-intolerance
• diarrhoea, hyperemesis
• gritty eyes, diplopia, tears

General signs:

• tremor
• hot & damp skin, alopecia
• proximal myopathy (respiratory muscle involvement not reported)
• eye signs such as stare, lid-lag & proptosis
• (severe opthalmic disease occurs only occasionally: marked proptosis, retro-orbital pressure, opthalmoplegia, papilloedema)

Cardiovascular signs:

• CVS is the most important organ-involvement in thyroid disease
• increased HR: S. Tachy or AF (AF may be the only sign, esp. in the elderly; hence the need for TFTs in any new AF)
• increased CO and pulse pressure
• precipitation of IHD, cardiac failure

B. Anti-Hyperthyroid Therapy

1. Medical Therapy

a) Beta-blockade:

• most rapid method of reversing symptoms
• effective within 12 - 24 hrs
• may inhibit peripheral conversion of T4 to T3 as well as blocking beta catech-olamine receptors
• usually only used to tide over while other therapies take effect

b) Methimazole/Carbimazole

• carbimazole is the prodrug of methimazole
• iodinated molecule blocks iodination of tyrosine residues
• effects seen after 3 - 4 weeks
• can be used as the sole therapy for hyperthyroidism: given for a period of 12 -18 mths but relapse rate >50%
• SFx - rash, arthralgia, N&V
• agranulocytosis: reversable

c) Propylthiouracil

• mechanism of action: a) as for carbimazole and b) blocks peripheral conversion of T4 to T3
• faster onset of action cf carbimazole (due to 'b' above)
• SFx same as carbimazole; can convert from one drug to the other if SFx a problem

d) Ablative Therapy

• Radioactive Iodine (I131)
• I131 concentrates in the thyroid and destroys functioning cells
• takes 6 -10 weeks for clinical effect
• repeat doses often necessary
• hypothyoidism can occur up to years after therapy
• aside from hypothyroidism, few side effects
• pregnancy an absolute contraindication
• no evidence for inherited genetic damage in babies if mother has had therapy in the past

2. Surgery

Due to I131, surgery for hyperthyroidism is less commonly required now than in the past. Subtotal thyroidectomy attempts to preserve the correct amount of tissue to allow euthyroid state post-op. Complications include:

• hypo- (or occasionally hyper-) thyroidism
• hypoparathyroidism

2. Thyroid Storm

Life-threatening exacerbation of hyperthyroidism. Onset is usually abrupt and precipitated by stress; eg surgery, infection.

Clinical features:

• fever, CO2 production, acidosis, hyperventilation
• CVS: tachycardia, arrythmia, CCF, shock
• CNS: agitation, tremor, delerium, coma
• GIT: diarrhoea, abdo. pain, vommiting

Management

• medical emergency; cannot wait for laboratory confirmation
• 02, active cooling (not aspirin as it displaces T4 from TBG)
• Beta-blockade: I.V. Propanolol 1-5 mg promptly treats fever, tachycardia, tremor; does not reduce O2 consumption
• Care with Beta-blockers if heart failure (could try esmolol)
• Glucocorticoid Rx often recommended due to possible "adrenocortical exhaustion"
• Iodide IV (as KI 60mg bd or NaI 1.0 - 2.5 g) rapidly controls thyrotoxicosis
• Can use lithium if allergic to iodide
• Oral antithyroid drugs commenced as soon as possible

DDx

• Phaeochromocytoma (Beta-blockade alone contraindicated)
• Malignant hyperthermia
• Dantrolene has been reported as useful in controlling hyperthermia and increased VO2 in thyroid storm

3. Hypothyroidism

Incidence of occult hypothyroidism 5%

A. Causes

• iatrogenic: following Rx for thyrotoxicosis, goitre or thyroid malignancy (TSH)
• primary: usually related to inflammatory disease of the thyroid (TSH)
• secondary: pituary insufficiency (innapropriately low TSH)

B. Manifestations

• sensitivity to cold, weight gain, deafness
• ammenorrhoea, anaemia, dry skin, sparse & lustreless hair
• myxoedema: accumulation of mucopolysaccharides in the tissues
• pale skin, puffy eyelids, enlarged tongue, thick lips, husky voice
• CVS: bradycardia, HS on CXR, heart failure (may cause tachycardia), pericardial effusion. Often have coronary artery disease. ECG: low voltage and /or ischaemia
• Neuro.: slowed mentation, psychosis, nerve compression due to myxoedema
• myxoedema coma: coma, hypothermia, CCF
• GIT: constipation, delayed gastric emptying
• hypothermia
• children: cretenism

C. Management

• Thyroxine 50 - 200 ug/day
• IV T3 in emergency
• Caution with thyroxine replacement if IHD because AMI has been precipitated by treatment of hypothyroidism: add beta-blocker early and keep to low dose of thyroxine
• Caution with digoxin dose in CCF as increased contractility may provoke ischeamia
  

4. Diffuse Goitre

A. Iodine Deficiency (Endemic Goitre)

Thyroid gland responds by increasing uptake of iodine. Chronic severe deficiency can cause goitre, which is:

• usually diffuse
• increased TSH causes glandular hypertrophy
• can become nodular with age
• can be massive and/orretrosternal
• uncommon in "developed" countries but common indication for thyroid surgery elsewhere

B. Simple Goitre

• ideopathic diffuse goitre in euthyroid patients
• surgery for cosmetic reasons

C. Other

Other causes of diffuse goitre include:

• rare inherited disorders of various parts of synthetic & control pathways
• Drugs which block iodine uptake, iodination of tyrosine or secretion of hormone, eg: Li-, I-, thiocynate, some sulphonylureas & sulphonamides

5. Nodular Goitre

Nodules vary from 1 to 200 mm. Patient can present with pressure effects (dysphagia, cough, voice change, dyspnoea), but usasually present as a lump.

• usually multinodular
• usually euthyroid and benign
• may develop hyperthyroidism with age
• may spread retrosternally
• some regress with thyroxine replacement
• other Rx includes I131 and subtotal thyroidectomy

6. Iodine Excess

• excess iodine inhibits iodination of thyroglobulin (Wolff Chaikoff effect)
• very high levels may also inhibit thyroid hormone release
• chronic iodine excess can cause goitre

7. Chronic Illness

• severe systemic illness causes decreased peripheral conversion T4 to T3
• decreaesd TBG may accompany hypoproteinaemia
• TSH remains normal

8. Malignant Thyroid Disease

• most thyroid cancers are slow-growing & have good prognosis
• medullary Ca
• more likely to invade and to metastasize than follicular & papillary Ca
• part of MEN (along with phaeochromocytoma, parathyroid adenoma & neurofibromata)
• management is usually surgery followed by suppresion with thyroxine and/or I131
• DXRT may be used for medulalry & anaplastic tumors
• lymphomas occur in the thyroid and may be accompanied by lymphoma elsewhere (such as the mediastinum)

9. Thyroiditis

A. Hashimoto's Thyroiditis

• antithyroid antibodies & lymphocytic infiltration of the gland
• firm, moderately large goitre
• may cause compressive symptoms (mild)
• typically "up, down, normal" course; ie transient hyperthyroidism followed by hypothyroidism which usually resolves eventually
• management is with thyroxine to suppress gland and/or steroids (surgery rarely indicated)

B. Subacute Thyroiditis (de Quervain's)

• viral illness (Coxsackie B)
• painful
• management similar to Hashimoto's

INVESTIGATIONS

1. TSH

TSH alone is now the initial lab test of thyroid function. High TSH has long been the hallmark of hypothyroidism. Previous assays gave immeasurable TSH in 10-30% of normals. Newer assay is accurate at lower concentrations, therefore low TSH is now adequate as a screen for hyperthyroidism

2. Free T3 & Free T4

Radio-immunoassays allow direct measurement, therefore now no need for measuring thyroid hormone binding capacity. Similarly, thyroxine-binding globulin level no longer important.

3. Anti-Thyroid Antibodies

4. Thyroid Scan

Uptake of I131 by functional thyroid tissue imaged by gamma camera. Cold nodules = increased likelihood of cancer.

5. Ultrasound

Useful for distinguishing cystic from solid nodules

6. FNAB

7. CXR

May show a a restrosternal mass, tracheal deviation or lung secondaries.

8. CT Scan

May deliniate retrosternal spread, detect metastastic disease, aid diagnosis, eg lymphoma, and better define airway anatomy than the CXR.

ANAESTHETIC IMPLICATIONS

1. Hyperthyroidism

Except for absolute emergency surgery, all patients should be clinically euthyroid prior to surgery.

Pharmacological stabilisation of hyperthyroid patient requires at least 6 -8 weeks. Beta-blockade combined with iodide (or lithium) can achieve euthyroid state in 1 -2 weeks but cardiac effects take longer to resolve.

• risk of thyroid storm provoked intraop or, more frequently, postop
• other risks of hyperthyroidism:
• cardiac failure
• increased sensitivity to catecholamine-induced arrhythmias

Emergency Surgery in hyperthyroid patient

• commence anti-thyroid Rx as soon as diagnosis made (in conjunction with specialist endocrinologist)
• preop sedation, eg with benzodiazepine
• in the days before medical stabilisation preop was rigidly adhered to, they talked about "stealing" the patient from the ward: unknown to the patient, a barbituate was added to their IV fluids and the drowsy patient was then stolen away to the operating theatre!

  Intraoperatively:

  • avoid sympathetic stimulation, eg ketamine, pancuronium, adrenaline in LA
  • continue beta-blockade titrated to heart rate
  • consider regional technique to decreased symp. stimulation
  • monitor HR, Temp, IBP, ETCO2, SpO2, ABGs
  • may have increased inhalational anaesthetic requirement due to increased cardiac output, increased temperature, ? CNS excitation
  • care with exopthalmic eyes

  Postoperatively:

  • intensive monitoring

   

2. Hypothyroidism

Less evidence that uncorrected hypothyroidism confers higher perioperative risk. (Evidence supports the safety of CABG prior to the correction of hypothyroidism.)

Anaesthetic Considerations:

• increased risk of hypothermia, hypoglycaeemia, hyponatreania, anaemia, hypovolaemia, adrenocortical insufficiency
• possibly increased sensitivity to sedatives - usually little or no sedative premed is needed
• possibility of prolonged recovery
• ?reduced anaesthetic requirements
• reduced ventilatory response to hypoxia/hypercarbia
• support ventilation intraop
• monitor ventilation postop
• gastric paresis therefore always consider RSI
• consider regional technique to avoid sedation but possibly reduced drug metabolism so care with dose of LA
• impaired myocardial response to reduced afterload
• maintain body heat

3. Thyroid Surgery

A. Preoperative Assessment:

1. Gland Function - is the patient clinically euthyroid?

• most important indicator of adequacy of medical preparation is resolution of symptoms, weight gain & normal heart rate
• assess cardiac status
• history & examination
• investigations: CXR, ECG, other Ix of function &/or ischaemia as indicated
• review investigations, esp. recent TFTs
• keep in mind possible associated conditions; myaesthenia gravis & rheumatoid arthritis with Graves's disease and phaechromocytoma with medullary Ca of the thyroid
• check Chvostek's & Trousseau's Signs preop (to exclude false +ve postop)

2. Airway

Rarely, goitre can cause difficulty with intubation; cancer rarely invades the larynx, trachea or recurrent laryngeal nerves.

In addition to usual assessment of airway, note:

• SOB, stridor or dysphagia may indicate significant mass effect
• review CXR &/or CT for evidence of tracheal deviation or compression
• flow-volume loops demonstrate any obstruction to air flow
• indrect laryngoscopy can aid preop assessment of the upper airway
• prepare the patient for awake intubation if appropriate
• enlarged tongue in myxoedema may add to airway difficulty
• lingual thyroid has been reported to interfere with laryngoscopy
• goitre may make emergency tracheostomy difficult
• neck mass may preclude percutaneous anaesthesia of the sup. laryngeal nerves and percutaneous passage of a wire for retrograde intubation

3. Other

Retrosternal spread can cause SVC obstruction:

• airway oedema
• dependance on spontaneous respiration for venous return
• haemodynamic instability

If the goitre is due to lymphoma then there may be anterior mediastinal lymphadenopathy with the possibility of cardiac compression.

B. Preparation of the Patient

• G&H
• continue thyroid medications
• if euthyroid then no particular considerations for premedication

C. Intraoperative management

GA is usual, but thyroidectomy can also be performed with bilateral cervical plexus blocks or accupuncture.

1. Monitoring

• standard including ETCO2 and Temp

2. Induction

• any induction agent appropriate (except perhaps ketamine if risk of hyperthyroidism)
• thiopentone has some anti-thyroid activity but insignificant
• large-bore cannula

3. Airway

Retricted access to the airway once surgical drapes in place!

• ETT mandatory
• some recommend armoured ETT
• nasal RAE works well
• place the cuff beyond the point of extrinsic compression

4. Position

• supine, padding behind shoulders to extend the neck

D. Postoperative Management:

1. Airway problems

The most important postop concern; usually require extreme care with airway management on induction (surgeon present, tracheostomy equipment available, strong preference for awake fibreoptic intubation). Difficulty with respiration may be due to:

a) Haematoma; either immediate or delayed

• airway obstruction is due more to lymphatic obstruction than to the direct effects of the haematoma therefore relief of the haematoma does not immediately relieve the obstruction
• give O2 and/or CPAP
• remove sutures & open the wound
• If intubation required for obstruction then extubate only when leak around the ETT and adequate facility & personel for reintubation

b) Recurrent laryngeal nerve palsy

Unilateral neuropraxia not uncommon and results in voice change and slight stridor (abductor fibres more at risk).

Bilateral palsy rare:

• 1/30,000
• flaccid cords are drawn together by the Bernouli effect during inspiration
• causes complete obstruction requiring reintubation and tracheostomy
• it used to be taught to extubate the patient deep to allow evaluation of vocal cord movement by direct laryngoscopy but this is becoming less popular

c) Tracheomalacia

• the tracheal rings can become weakened by the effects of prolonged pressure due to large goitre
• postoperative collapse of the trachea can cause complete obstruction
• rare except in areas of endemic goitre with late presentation

2. Tetany

• hypocalcaemia typically develops 24 -72 hrs postop
• surgeon attempts to identify and preserve the parathyroid glands but this can be difficult
• hypoparathyroidism can present as laryngospasm
• monitor serum Ca++ & Mg++
• Chvostoek's sign: facial muscle spasm on tapping of the facial nerve
• Trousseau's sign: carpopedal spasm within 4 mins of inflating a blood pressure cuff to >systolic BP

Management

• Mg++ promotes PTH release
• Ca++ given immediately if clinically detectable hypocalcaemia

READING LIST:

• Oxford Textbook of Medicine
• Davidson's Principles and Practice of Medicine
• Stoelting RK, Dierdorf SF. Anaesthesia and Co-Existing Disease 3rd ed. New York: Churchill Livingstone, 1993: 347-54
• Katz J, Benemof JL, Kadis L. Anaesthesia and Uncommon Diseases 3rd ed. Philadelphia: W.B. Saunders, 1990: 254-61
• Todesco J, Williams RT,Eagle CJ Anaesthetic management of a patient with a large neck mass. Can J Anaesth 1994; 41:157-60
• Kho HG, et al Accupuncture anaesthesia. Observations on its use for removal of thyroid adenomata and influence on recovery and morbity in a Chinese hospital. Anaesthesia 1990; 45:480-5
• Bennett MH, Wainwright AP Acute thyroid crisis on induction of anaesthesia. Anaesthesia 1989; 444:28-30