4.2 Thyroid Disorders: Thyrotoxicosis, Hypothyroidism & Thyroid Storm

Etiologies & Differential Diagnosis of Thyrotoxicosis

Thyrotoxicosis encompasses the clinical syndrome resulting from excessive circulating free thyroid hormones (triiodothyronine [FT3] and thyroxine [FT4]), regardless of source. Hyperthyroidism refers specifically to de novo hyperfunctioning of the thyroid gland itself. Correctly identifying the underlying etiology is crucial because clinical management ranges from targeted antithyroid pharmacotherapy to radioactive iodine ablation, surgery, or simple watchful waiting.

Primary Hyperthyroidism Etiologies

1. Graves' Disease: The most prevalent cause of hyperthyroidism worldwide and across the Caribbean. An autoimmune disorder characterized by circulating TSH-receptor autoantibodies (TRAb / TSI) that bind and constitutively stimulate the G-protein-coupled TSH receptor, driving unregulated thyroid follicular hyperplasia and hormone synthesis. Extrathyroidal manifestations arise from autoantibodies cross-reacting with TSH receptors on retro-orbital and pretibial fibroblasts, stimulating glycosaminoglycan synthesis and adipogenesis:
   • Thyroid Orbitopathy: Proptosis, periorbital edema, chemosis, lid lag, and diplopia due to extraocular muscle enlargement.
   • Pretibial Myxedema (Thyroid Dermopathy): Non-pitting, violaceous, indurated plaques with a 'peau d'orange' texture on the anterior shins.
   • Thyroid Acropachy: Digital clubbing and periosteal new bone formation.
2. Toxic Multinodular Goiter (Plummer's Disease): Multiple autonomous hyperfunctioning follicular nodules arising against a background of chronic non-toxic goiter. Frequently seen in older adults and regions of past iodine deficiency. Extrathyroidal signs (orbitopathy, myxedema) are absent.
3. Toxic Solitary Adenoma: A single autonomous monoclonal adenoma harboring somatic gain-of-function mutations in the TSH receptor or Gs-alpha gene (GNAS). The remainder of the thyroid gland is suppressed and atrophic on scintigraphy.
4. Subacute de Quervain's Thyroiditis (Granulomatous Thyroiditis): A self-limiting post-viral inflammatory condition. Follicular cells are destroyed, releasing preformed stored colloid into the circulation. Classically presents with an exquisitely tender, painful enlarged thyroid gland radiating to the jaw or ears, preceding upper respiratory symptoms, fever, and markedly elevated erythrocyte sedimentation rate (ESR > 50-100 mm/h) and C-reactive protein (CRP). Displays a characteristic triphasic clinical course: transient thyrotoxicosis (2-6 weeks), followed by transient subclinical/overt hypothyroidism (several weeks), progressing to complete euthyroid recovery in > 90% of patients.
5. Other Causes: Silent/postpartum thyroiditis, factitious thyrotoxicosis (surreptitious levothyroxine ingestion, marked by suppressed serum thyroglobulin), struma ovarii (teratoma containing functional thyroid tissue), and amiodarone-induced thyrotoxicosis (Type 1: iodine-induced in underlying multinodular goiter; Type 2: destructive thyroiditis).

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Laboratory & Scintigraphic Evaluation

Initial diagnostic evaluation relies on third-generation serum TSH and free thyroid hormones:

• Primary Hyperthyroidism: Suppressed serum TSH (< 0.01 mU/L) with elevated Free T4 and/or Free T3.
• T3-Toxicosis: Suppressed TSH with elevated Free T3 and normal Free T4 (seen in early Graves' or toxic adenomas; carries higher risk of cardiac arrhythmias).
• Subclinical Hyperthyroidism: Suppressed TSH with normal Free T4 and Free T3.
• Secondary (Central) Hyperthyroidism: Elevated or inappropriately normal TSH with elevated Free T4/T3 (due to a TSH-secreting pituitary adenoma or thyroid hormone resistance).

Radioactive Iodine Uptake (RAIU) & Scintigraphy Patterns

• Diffusely Elevated Uptake (40% to 80%): Graves' disease (homogeneous global tracer uptake).
• Patchy, Heterogeneous Uptake: Toxic multinodular goiter (multiple hot foci interspersed with suppressed cool background).
• Single Intense 'Hot' Focus: Toxic solitary adenoma (intense uptake localized to the nodule, with complete suppression of the contralateral lobe).
• Suppressed / Negligible Uptake (< 1% to 2%): Subacute de Quervain's thyroiditis, silent/postpartum thyroiditis, factitious thyrotoxicosis, amiodarone Type 2, and recent exogenous iodinated contrast load.

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Thyroid Function Diagnostic Interpretation

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Thyroid Storm (Thyrotoxic Crisis)

Thyroid storm represents an extreme, life-threatening hypermetabolic state resulting from severe decompensation of thyrotoxicosis. In-hospital mortality historically ranges from 10% to 30%.

Precipitating Triggers

Rarely occurring spontaneously, thyroid storm is typically ignited by an acute physiological stressor in a patient with underlying uncontrolled thyrotoxicosis: systemic infections/sepsis, non-compliance with antithyroid medication, thyroid or non-thyroid surgery, trauma, labor and delivery, diabetic ketoacidosis, severe emotional distress, or sudden exposure to iodinated radiocontrast agents.

Clinical Manifestations & Diagnostic Scoring

• Thermoregulatory Failure: Marked hyperpyrexia, typically 38.5°C to > 40.5°C (101°F to 106°F), accompanied by profuse diaphoresis.
• Cardiovascular Collapse: Tachycardia markedly out of proportion to temperature (140 to 180 bpm), atrial fibrillation, wide pulse pressure, high-output congestive heart failure, and cardiogenic shock.
• Central Nervous System Dysfunction: Agitation, emotional lability, delirium, acute psychosis, visual hallucinations, chorea, seizures, stupor, and progression to frank coma.
• Gastrointestinal / Hepatic Toxicity: Nausea, intractable vomiting, profuse watery diarrhea, severe abdominal pain, hepatomegaly, and jaundice (a grim prognostic sign reflecting centrilobular hepatic necrosis).
• The Burch-Wartofsky Point Scale (BWPS): Quantifies organ dysfunction. A score >= 45 is highly suggestive of thyroid storm, 25-44 indicates impending storm, and < 25 indicates storm is unlikely. Management must begin immediately based on clinical suspicion without awaiting specialized laboratory assays.

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Multimodal Resuscitation and Specific Treatment

Begin supportive resuscitation, treatment of the precipitant, thionamide and glucocorticoid therapy promptly. Assess cardiac function and haemodynamics before beta blockade. These treatments overlap; the critical drug-order rule is to give iodine at least 1 hour after thionamide.

Beta-Adrenergic Antagonism

• Propranolol: In haemodynamically stable adults without significant LV dysfunction, 60–80 mg orally every 4–6 hours is a recognized storm regimen (or 1–2 mg slowly IV over 10 minutes with ECG monitoring). Start cautiously if cardiac dysfunction is suspected; high-dose beta blockade can precipitate collapse. In ICU, titratable short-acting esmolol may be preferred, with close haemodynamic monitoring.
• Dual Action: Blocks systemic beta-1 and beta-2 adrenergic hyperstimulation (reducing heart rate, tremor, and anxiety) and, at high doses (> 160 mg/day), inhibits peripheral 5'-monodeiodinase, blunting peripheral conversion of T4 to active T3. (Esmolol continuous IV infusion is an alternative in intensive care units; cardioselective beta-blockers like metoprolol can be substituted in patients with reactive airway disease).

Inhibition of New Hormone Synthesis (Thionamides)

• Propylthiouracil (PTU): Administer 500 to 1000 mg loading dose orally or via nasogastric tube, followed by 250 mg every 4 hours.
• Why PTU is Preferred in Storm: PTU inhibits thyroid peroxidase (blocking de novo organification and coupling) AND inhibits peripheral conversion of T4 to T3. Methimazole (20 mg orally every 4-6 hours) inhibits synthesis but does NOT inhibit peripheral conversion. (Note: In routine ambulatory Graves' disease, methimazole is preferred due to PTU's risk of fulminant hepatic necrosis, except during the first trimester of pregnancy where PTU is used to avoid methimazole-associated embryopathy/aplasia cutis).

Blockade of Hormone Release (Inorganic Iodine)

• Administer Lugol's iodine (8 to 10 drops orally every 6 to 8 hours) or Saturated Solution of Potassium Iodide (SSKI, 5 drops every 6 hours).
• THE CRITICAL TIMING RULE: Iodine solutions must be administered AT LEAST 1 HOUR AFTER the initial thionamide dose. Administering inorganic iodine before or simultaneously with thionamide delivers an enormous iodide substrate load to an uninhibited gland, fueling massive new thyroid hormone synthesis (the Jod-Basedow effect). When given after thionamide block, high iodide concentrations rapidly halt the release of preformed T4 and T3 from thyroglobulin via the acute Wolff-Chaikoff effect.

Corticosteroid Therapy

• Start glucocorticoids with the initial treatment; do not wait for iodine. Administer Intravenous Hydrocortisone 100 mg IV every 8 hours (or Dexamethasone 2 mg IV every 6 hours).
• Rationale: Glucocorticoids directly inhibit peripheral T4-to-T3 conversion, stabilize vascular endothelial tone, and provide empiric coverage for relative adrenal insufficiency caused by accelerated hepatic cortisol breakdown in severe thyrotoxicosis.

Bile Acid Sequestrants & Supportive Care

• Cholestyramine: 4 g orally four times daily binds thyroid hormones in the intestinal lumen, interrupting the enterohepatic circulation and accelerating hormone clearance.
• Hyperpyrexia Management: Aggressive active cooling with cooling blankets and ice packs. Prescribe acetaminophen. AVOID ASPIRIN AND SALICYLATES, which displace T4 and T3 from thyroid-binding globulin (TBG), dramatically elevating free active hormone fractions.

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Hypothyroidism & Myxedema Coma

Primary Hypothyroidism

• Hashimoto's Thyroiditis (Chronic Autoimmune Thyroiditis): The predominant etiology in iodine-replete regions. Characterized by lymphocytic infiltration, Hurthle cell transformation, and high titers of anti-thyroid peroxidase (anti-TPO) and anti-thyroglobulin (anti-Tg) autoantibodies.
• Clinical Features: Fatigue, cold intolerance, weight gain, constipation, dry coarse skin, loss of outer third of eyebrows (madarosis), bradycardia, non-pitting periorbital and pretibial edema (myxedema caused by hyaluronic acid accumulation), carpal tunnel syndrome, and characteristic delayed relaxation phase of deep tendon reflexes ('hung-up' reflexes).
• Levothyroxine (L-T4) Therapy: Standard replacement dose is 1.6 mcg/kg/day in young, healthy adults. In elderly patients or those with documented coronary artery disease, initiate at a low dose (25 to 50 mcg/day) and titrate upward slowly every 6 to 8 weeks based on serum TSH to prevent precipitating myocardial ischemia, angina, or atrial fibrillation. In pregnant women with established hypothyroidism, the levothyroxine dose must be increased by 25% to 30% immediately upon pregnancy confirmation to ensure adequate fetal neurodevelopment.

Myxedema Coma

Myxedema coma is the life-threatening decompensation of severe, long-standing untreated hypothyroidism, carrying a mortality rate up to 30% to 50%.

• Classic Clinical Triad: (1) Severe Hypothermia (core temperature frequently < 35.0°C / 95°F, sometimes < 30°C), (2) Altered Mental Status (stupor, lethargy, or coma), and (3) Hypoventilation / Bradycardia (hypercapnic respiratory failure and profound sinus bradycardia).
• Emergency Management:
  1. Passive External Rewarming: Cover with regular blankets in a warm room. Caution: Active external warming (heating blankets) causes peripheral vasodilation and precipitates catastrophic cardiovascular collapse and refractory shock.
  2. Stress-Dose Glucocorticoids: Administer Intravenous Hydrocortisone 100 mg IV stat, then 50-100 mg every 8 hours. Mandatory Principle: Corticosteroids must be given BEFORE or CONCURRENTLY with thyroid hormone. Giving thyroid hormone alone accelerates metabolic clearance of cortisol, precipitating fatal adrenal crisis in patients with unrecognized concurrent adrenal insufficiency.
  3. Intravenous Thyroid Hormone: Administer Levothyroxine loading dose of 200 to 400 mcg IV over 30 minutes, followed by 50 to 100 mcg IV daily until oral administration is feasible. (Cautious addition of IV liothyronine [T3] 5-10 mcg every 8 hours may be considered in severe coma).
  4. Supportive Resuscitation: Early endotracheal intubation for hypoventilation and coma, cautious fluid resuscitation, and correction of severe hyponatremia with fluid restriction or hypertonic (3%) saline if symptomatic seizures occur.