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Thyrotoxicosis - Complete PG Medicine Teaching + LAQ Guide

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QUICK CONCEPTUAL FRAMEWORK (Read this first)

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LONG ANSWER QUESTION FORMAT

"Discuss Thyrotoxicosis - Etiology, Pathogenesis, Clinical Features, Diagnosis, and Management including Thyroid Storm"

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I. DEFINITION

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II. ETIOLOGY / CLASSIFICATION

A. Thyrotoxicosis WITH Hyperthyroidism (increased synthesis)

B. Thyrotoxicosis WITHOUT Hyperthyroidism (release of stored hormone)

• Subacute (de Quervain) thyroiditis - painful, post-viral
• Silent/postpartum thyroiditis - painless, autoimmune
• Drug-induced thyroid destruction: amiodarone, cytokines, tyrosine kinase inhibitors, immune checkpoint inhibitors (nivolumab)
• Radiation thyroiditis
• Infarction of adenoma
• Thyrotoxicosis factitia (exogenous ingestion)

C. Secondary Hyperthyroidism (TSH-driven)

• TSH-secreting pituitary adenoma
• Thyroid hormone resistance syndrome
• Chorionic gonadotropin-secreting tumors (gestational thyrotoxicosis, hydatidiform mole)

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III. GRAVES' DISEASE - PATHOGENESIS (Most important - max marks)

1. Loss of immune tolerance to TSH receptor (TSH-R)
2. B-lymphocytes in thyroid, bone marrow, and lymph nodes produce thyroid stimulating immunoglobulins (TSI)
3. TSI binds TSH-R and continuously stimulates thyroid follicular cells - mimicking TSH action
4. Results in diffuse thyroid hyperplasia + unregulated hormone synthesis
5. TSH-R antibodies (TRAb) detectable by immunoassay - cross placenta and cause neonatal thyrotoxicosis
6. Concurrent TPO and Tg antibodies in up to 80% of cases

• Activated T cells infiltrate extraocular muscles
• IFN-γ, TNF, IL-1 activate fibroblasts
• Increased glycosaminoglycan synthesis traps water → muscle swelling
• IGF-1R aberrant signaling on orbital fibroblasts (basis for teprotumumab therapy)
• Late stage: irreversible fibrosis → diplopia

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IV. CLINICAL FEATURES

A. General Features of Thyrotoxicosis (any cause)

B. Features Specific to Graves' Disease (the triad)

1. Diffuse goiter - smooth, non-tender, bruit may be audible (due to high vascularity)
2. Ophthalmopathy (Graves' orbitopathy)
   • Periorbital edema, chemosis, proptosis/exophthalmos
   • Lid retraction (Dalrymple's sign), lid lag (von Graefe's sign)
   • External ophthalmoplegia, diplopia
   • Corneal ulceration, optic neuropathy (vision-threatening)
3. Dermopathy (pretibial myxedema)
   • Non-pitting, indurated skin thickening over shins
   • Orange-peel texture ("peau d'orange")
4. Thyroid acropachy (rare) - clubbing + periosteal new bone formation

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V. INVESTIGATIONS / DIAGNOSIS

Step-by-step (follow the algorithm):

• TSH suppressed + T4 elevated = Primary thyrotoxicosis (most cases)
• TSH suppressed + T4 normal → check free T3 → if elevated = T3 toxicosis (seen in early Graves', toxic adenoma)
• TSH normal/elevated + T4 elevated = TSH-secreting pituitary adenoma or thyroid hormone resistance

• TRAb (TSH receptor antibody): Positive in Graves' disease - most specific test
• Radionuclide scan (99mTc, 123I, 131I):
  • Diffuse high uptake = Graves' disease
  • Focal hot nodule + suppressed surrounding = Toxic adenoma
  • Low/patchy uptake = Destructive thyroiditis or factitial
• Color-flow Doppler USG: High blood flow = hyperthyroidism; low flow = thyroiditis (avoids radioactivity)

• Elevated bilirubin, liver enzymes, ferritin (common in thyrotoxicosis)
• Microcytic anemia, thrombocytopenia may occur
• ECG: sinus tachycardia, AF
• In T4 toxicosis: iodine excess providing excess substrate → elevated T4 with normal T3

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VI. MANAGEMENT OF THYROTOXICOSIS

Three main modalities:

A. Antithyroid Drugs (ATDs)

• Methimazole/Carbimazole: 10-20 mg every 12 h initially; once-daily dosing after euthyroidism
• PTU: 100-200 mg every 6-8 h (always in divided doses)
• Lower doses in iodine-deficient areas

• Titration regimen (preferred): Gradually reduce dose as thyrotoxicosis improves - minimizes drug dose, monitors response
• Block-replace regimen: High-dose ATD + LT4 added to prevent hypothyroidism - avoids dose adjustment, higher drug exposure

• Minor (1-5%): rash, urticaria, fever, arthralgia
• Major (rare): Agranulocytosis (<1%) - most important; hepatitis (PTU > methimazole); cholestasis (methimazole); vasculitis
• Warn all patients: sore throat, fever, mouth ulcers = stop drug immediately, check CBC

B. Beta-Blockers (symptomatic control)

• Propranolol 20-40 mg every 6 h OR atenolol (longer-acting, selective β1)
• Control adrenergic symptoms (palpitations, tremor, anxiety) while ATDs take effect
• Also useful in thyrotoxic periodic paralysis

C. Radioactive Iodine (RAI - 131I)

• Definitive treatment for Graves', toxic MNG, toxic adenoma
• Contraindicated in: pregnancy, active/severe ophthalmopathy (worsens eye disease, especially smokers)
• May precipitate thyroid storm: ensure euthyroid state before RAI; rarely due to hormone release from damaged follicles
• Hypothyroidism is the expected outcome (15-30% at 1 year, up to 80% at 10 years) - actually desired

D. Surgery (Total/Near-Total Thyroidectomy)

• Indications: large goiter, compressive symptoms, malignancy suspected, patient preference, failed ATDs
• Advantages: rapid cure, suitable in pregnancy (2nd trimester preferred)
• Complications: recurrent laryngeal nerve palsy, hypoparathyroidism

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VII. THYROID STORM (Thyrotoxic Crisis)

Definition:

Precipitants (MNEMONIC: "SITS WIDE"):

• Surgery (thyroid or non-thyroid)
• Infection (most common precipitant)
• Trauma
• Stress (physical/emotional)
• Withdrawal of antithyroid drugs
• Iodine excess / RAI administration
• DKA / hyperosmolar coma
• Eclampsia / labor / delivery; MI, PE, CVA

Clinical Features:

• Fever (>38.5°C) - hallmark
• Tachycardia (often >140 bpm), AF, high-output cardiac failure
• CNS: Agitation → delirium → psychosis → seizures → coma
• GI: Nausea, vomiting, diarrhea, abdominal pain, jaundice (severe - marker of bad prognosis)
• Diaphoresis, vomiting, dehydration

Diagnosis - Burch-Wartofsky Point Scale (BWPS):

• ≥45: Thyroid storm
• 25-44: Impending storm - treat aggressively
• <25: Unlikely storm

Treatment of Thyroid Storm (Stepwise - must memorize the ORDER):

• ICU admission
• IV fluids, cooling blankets, paracetamol (avoid aspirin - displaces T4 from TBG)
• Treat arrhythmias

• Propranolol IV: 0.5-1 mg/min slow IV up to 10 mg; OR 60-80 mg PO every 4-6 h
• Alternatively: esmolol infusion (titratable, short-acting - preferred in cardiac compromise)

• PTU 500-1000 mg loading dose PO, then 250 mg every 4 h (preferred over methimazole in storm - also blocks T4→T3 conversion)
• OR Methimazole 20-40 mg PO every 6-8 h (if PTU unavailable)

• Lugol's iodine: 8-10 drops PO every 6-8 h
• OR SSKI (saturated KI): 5 drops every 6 h
• OR Lithium carbonate 300 mg every 6 h (if iodine allergy or agranulocytosis)
• (Must give ATDs FIRST - otherwise iodine provides substrate for more hormone synthesis - "Wolff-Chaikoff vs Jod-Basedow paradox")

• Hydrocortisone 300 mg IV bolus, then 100 mg IV every 8 h (also corrects relative adrenal insufficiency from hypermetabolism)
• OR Dexamethasone 2 mg IV every 6 h

• Cholestyramine 4 g every 6 h (binds TH in enterohepatic circulation)

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VIII. SPECIAL SITUATIONS

Subclinical Hyperthyroidism

• Suppressed TSH, normal free T4 and T3
• Treat if: age >65 years, AF, osteoporosis risk, or TSH persistently <0.1

Thyrotoxic Periodic Paralysis

• More common in Asian males
• Episodic hypokalemia + muscle weakness
• Trigger: high carbohydrate meal, exercise
• Treat: propranolol + KCl replacement; definitive = treat thyrotoxicosis

Amiodarone-induced Thyrotoxicosis (AIT)

• Type 1: Jod-Basedow (iodine-induced excess synthesis) - treat with ATDs
• Type 2: Destructive thyroiditis - treat with prednisolone
• Distinguish by color Doppler (Type 1: high flow; Type 2: low/absent flow)

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IX. KEY DIFFERENTIALS TO REMEMBER FOR EXAM

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X. HIGH-YIELD EXAM POINTS FOR MUHS

1. PTU is preferred in thyroid storm (not methimazole) - because it also blocks T4→T3 conversion
2. Iodine must be given AFTER ATDs (1 hour gap) - not before
3. Avoid aspirin in thyroid storm - displaces thyroid hormone from TBG
4. BWPS ≥45 = thyroid storm confirmed
5. Teprotumumab - monoclonal antibody targeting IGF-1R for Graves' ophthalmopathy (new)
6. T3 toxicosis - suppressed TSH, normal T4, elevated T3 - seen in early Graves' and toxic adenoma
7. Apathetic hyperthyroidism in elderly - AF + weight loss without classic features
8. Agranulocytosis - most dangerous ATD side effect; onset idiosyncratic and abrupt; stop drug immediately if sore throat/fever
9. Block-replace vs titration - titration preferred to minimize drug exposure
10. Remission rate of ATDs: 30-60% at 18 months

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SAMPLE LAQ ANSWER OUTLINE (Exam format, 15 marks)

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