Acquired Hypothyroidism

Overview & Epidemiology

• Acquired hypothyroidism in adolescence most commonly arises from autoimmune etiology.
• Strong female predominance; Female:Male ratio 4-6:1.
• Peak incidence of autoimmune thyroiditis occurs during adolescence (11-15 years).

Etiology

Common & Important Causes

Category	Specific Causes	Pathophysiology & Notes
Autoimmune (Most Common)	Hashimoto Thyroiditis (Chronic Lymphocytic Thyroiditis)	Organ-specific autoimmune destruction. T-cell and B-cell infiltration into thyroid follicles. Progression to follicular fibrosis and glandular atrophy.
Iodine Related	Endemic Goiter (Deficiency)Iodine Excess	Severe deficiency limits hormone synthesis.Excess induces Wolff-Chaikoff effect (acute block in hormone release/synthesis).
Drug-Induced	Anticonvulsants, Lithium, Amiodarone, Tyrosine Kinase Inhibitors	Anticonvulsants (valproate, phenytoin) stimulate hepatic cytochrome P450, increasing T4 clearance. Amiodarone/Lithium disrupt synthesis/release.
Iatrogenic	Neck Irradiation, Surgery	Late consequence of mantle radiation 1 (Hodgkin lymphoma), total body irradiation (bone marrow transplant), or excision of thyroglossal duct cyst.
Central (Secondary)	Pituitary/Hypothalamic Lesions	Craniopharyngioma, head trauma, meningitis, or cranial irradiation disrupting TSH/TRH secretion.
Delayed Congenital	Ectopic Thyroid, Dyshormonogenesis	Mild defects or failing ectopic sublingual glands may maintain euthyroidism until adolescence, presenting as acquired disease.
Systemic/Infiltrative	Cystinosis, Langerhans Cell Histiocytosis	Glandular destruction via intralysosomal cystine storage or histiocytic infiltration.

High-Yield Syndromic Associations

Girls presenting with acquired hypothyroidism warrant screening for comorbid chromosomal or autoimmune conditions:

• Chromosomal:
  • Turner Syndrome (8-30% incidence),
  • Trisomy 21 (15-20% incidence).
• Autoimmune:
  • Type 1 Diabetes Mellitus,
  • Celiac disease,
  • Vitiligo,
  • Alopecia,
  • Autoimmune Polyglandular Syndromes (APS Type 1 and Type 2).

Laboratory Evaluation & Findings

Thyroid Function Tests (TFTs)

Crucial for differentiating primary versus central etiology. Must utilize age-specific pediatric reference ranges.

• Primary Overt Hypothyroidism: Low Free Thyroxine (FT4), Markedly Elevated TSH (>10 mU/L).
• Subclinical Hypothyroidism: Normal FT4, Mildly Elevated TSH.
• Central Hypothyroidism: Low FT4, Inappropriately Low, Normal, or mildly elevated TSH.

Autoantibody Markers

• Anti-Thyroperoxidase (TPO-Ab): Primary mediator of antibody-dependent cell-mediated cytotoxicity. Positive in vast majority of Hashimoto cases.
• Anti-Thyroglobulin (Tg-Ab): Often present, though less pathologically destructive.
• Clinical Utility: Titers confirm autoimmune etiology but do not correlate with disease severity or thyroid function. Serial monitoring not recommended.

Ancillary Laboratory Findings

Long-standing or severe hypothyroidism induces systemic metabolic derangements:

• Lipid Profile: Hypercholesterolemia, dyslipidemia.
• Hematology: Macrocytic anemia.
• Metabolic/Electrolytes: Hyponatremia.
• Muscle: Elevated creatine phosphokinase (CPK).
• Urine: Urinary iodine concentration assesses suspected iodine deficiency or excess.

Management Principles

• Pharmacotherapy: Oral Levothyroxine (L-T4) is treatment of choice.
• Age-Specific Dosing: 10-16 years require 2-4 mcg/kg/day.
• Administration: Empty stomach. Separate from iron or calcium supplements by 6 hours to prevent absorption interference.
• Monitoring: Re-evaluate TSH and FT4 every 4-6 weeks after dose adjustments, then every 4-6 months. Target TSH within age-specific reference range.
• Severe Disease Precaution: In profound, long-standing hypothyroidism, initiate L-T4 at 1/3 to 1/2 conventional dose. Gradual up-titration prevents precipitating pseudotumor cerebri.

Footnotes

1. Mantle radiation is a specialized technique used in radiation therapy to treat lymph nodes above the diaphragm, covering the neck, chest, and armpits in a shape resembling a “mantle” or cape. It was historically a standard treatment for Hodgkin lymphoma to target multiple lymph node groups simultaneously while using lead blocks to shield the lungs and heart. ↩