Thyroid Disorders

Thyroid Physiology - Neck's Powerhouse

• Axis: Hypothalamus (TRH) → Pituitary (TSH) → Thyroid (T3/T4). Negative feedback.
• Synthesis: Iodide trapping → Oxidation → Organification (Thyroid Peroxidase - TPO) → Coupling → Release.
  • T4: Thyroxine (prohormone, major circulating); T3: Triiodothyronine (active, potent).
  • Peripheral conversion: $T_4 \rightarrow T_3$ by deiodinases (e.g., D1, D2).
• Transport: Bound to Thyroxine-Binding Globulin (TBG, ~70%), albumin.
• Actions: ↑Basal Metabolic Rate (BMR), essential for brain/somatic growth, ↑catecholamine sensitivity.

⭐ Wolff-Chaikoff effect: Excess iodide transiently inhibits TPO & organification, ↓T3/T4 synthesis.

Hypothyroidism - The Slowdown Show

Thyroid hormone deficiency → metabolic slowdown. Early Dx & Rx critical.

• Congenital Hypothyroidism (CH): Most common preventable cause of intellectual disability.
  • Screening: Neonatal, Day 3-5 (TSH ↑, T4/FT4 ↓). Treat by 2 weeks.
  • Causes: Dysgenesis (85%, e.g., ectopic thyroid), dyshormonogenesis.
  • Clinical: "Quiet baby", prolonged jaundice, macroglossia, umbilical hernia, hoarse cry, poor feeding, hypotonia.
• Acquired Hypothyroidism:
  • Causes: Hashimoto's thyroiditis (autoimmune, most common >6 yrs, goiter, +Anti-TPO Ab), iodine deficiency.
  • Clinical: Growth failure (↓ height velocity), ↓ school performance, cold intolerance, constipation, dry skin, lethargy.
• Diagnosis: ↑ TSH, ↓ T4/FT4 (primary).
• Treatment: Levothyroxine (e.g., CH: 10-15 µg/kg/day). Monitor TSH.

⭐ Neonatal screening for CH is crucial. Early levothyroxine (ideally within 2 weeks of birth) prevents irreversible intellectual disability.

Hyperthyroidism - The Overdrive Orchestra

• Etiology:
  • Graves' disease (most common): Autoimmune, TSH-Receptor Antibodies (TRAb) positive.
  • Neonatal thyrotoxicosis: Transplacental maternal TRAb; transient (3-12 weeks).
  • Rare: Toxic adenoma, McCune-Albright syndrome, iodine-induced.
• Clinical Features: "Thyroid on overdrive"
  • Weight loss (despite ↑appetite), heat intolerance, hyperactivity, anxiety, poor school performance.
  • Cardio: Tachycardia, palpitations, widened pulse pressure.
  • Goiter: Diffuse, firm (±bruit).
  • Graves' specific: Exophthalmos, lid lag, proptosis.
  • Growth: Accelerated linear growth, advanced bone age, later potential for short stature if untreated.
• Diagnosis:
  • ↓TSH, ↑Free T4 (FT4), ↑Free T3 (FT3).
  • TRAb positive in Graves' disease.
  • Thyroid Ultrasound: Diffuse goiter, ↑vascularity (thyroid inferno).
• Management:
  • Anti-thyroid drugs (ATDs):
    • Methimazole (MMI): Preferred. Initial dose 0.2-0.5 mg/kg/day (max 30 mg/day).
    • Propylthiouracil (PTU): For thyroid storm or MMI intolerance/allergy. Risk: severe hepatotoxicity.
  • β-blockers (e.g., Propranolol): Symptomatic relief (tachycardia, tremors).
  • Definitive Therapy (if ATDs fail/contraindicated): Radioactive Iodine (RAI) or Thyroidectomy.
• Thyroid Storm (Thyrotoxic Crisis):
  • Life-threatening: Hyperthermia, severe tachycardia, delirium, heart failure.
  • Rx: PTU (first), Iodides (Lugol’s/SSKI, ≥1hr after PTU), Propranolol, Corticosteroids. 📌 Mnemonic: PIPS (PTU, Iodides, Propranolol, Steroids).

⭐ In children, methimazole is the first-line antithyroid drug; PTU is reserved for specific situations like thyroid storm or MMI allergy/intolerance due to its black box warning for severe liver injury and vasculitis.

Thyroid Nodules & Goiter - Neck Check-up

• Goiter: Thyroid gland enlargement.
  • Types: Diffuse (smooth; e.g., Graves', early Hashimoto's), Nodular (lumpy).
  • Pediatric Causes: Iodine deficiency (endemic areas), Hashimoto's thyroiditis, Graves' disease, dyshormonogenesis.
• Thyroid Nodules (Pediatrics):
  • Less common vs. adults, but ↑ malignancy risk (~20-25%).
  • Risk Factors for Malignancy: Hx radiation, family Hx thyroid Ca, rapid growth, firm/hard, fixed, cervical LAD, hoarseness.
  • Evaluation: TSH, Ultrasound (USG).
  • USG High-Risk: Microcalcifications, irregular margins, taller-than-wide, hypoechoic.

  ⭐ Malignancy risk in pediatric thyroid nodules is ~20-25%, significantly higher than adults.

• Neck Examination:
  • Inspect: Swelling, symmetry, movement with swallowing.
  • Palpate: Size, consistency (soft, firm, hard), mobility, tenderness. Assess cervical lymph nodes.

High‑Yield Points - ⚡ Biggest Takeaways

• Congenital hypothyroidism (CH): most common preventable cause of intellectual disability; newborn screening (↑TSH, ↓T4) is vital.
• CH causes: worldwide iodine deficiency; iodine-sufficient areas: thyroid dysgenesis (ectopic most common).
• Juvenile hypothyroidism: presents with growth failure, declining school performance, and delayed bone age.
• Graves' disease: most common cause of pediatric hyperthyroidism; look for exophthalmos, goiter, ↑T4/T3, ↓TSH.
• Neonatal thyrotoxicosis: transient, in infants of mothers with Graves' disease (transplacental TSH-receptor antibodies).