Acquired hypothyroidism and Hashimoto thyroiditis — formative SAQs

SAQ 1 (10)

A 12-year-old girl presents with a six-month history of fatigue, a 4-kg weight gain, worsening constipation and declining school grades. Her height has crossed from the 50th to the 25th centile while her BMI has risen. Examination reveals a firm, bosselated, symmetrically enlarged, non-tender thyroid with no cervical lymphadenopathy. Venous TSH is 28 mU/L (reference 0.4–4.0), free T4 6 pmol/L (low) and anti-TPO antibodies strongly positive. ^[1][3]

1. Define acquired hypothyroidism, classify this case, and state the most likely cause and its diagnostic fingerprint. (3) ^[1][6]
2. Outline the immediate management plan, including the drug, dose calculation for a 40-kg child, route and timing, and the first recheck. (4) ^[2][7]
3. Describe the monitoring target and the long-term plan, including screening for associated conditions. (3) ^[2][4]

Model answer

Definition and classification. Acquired hypothyroidism is thyroid-hormone deficiency developing after a period of normal thyroid function. This is overt primary hypothyroidism (TSH raised + free T4 low) due to Hashimoto (chronic lymphocytic) thyroiditis, confirmed by anti-TPO antibodies (positive in ~90% — the diagnostic fingerprint). The firm bosselated goitre is typical. ^[1][6]

Immediate management. Start levothyroxine sodium once daily by mouth, weight-based. For a 12-year-old (>12-yr band, 2–3 mcg/kg/day): 40 kg × 2.5 mcg/kg ≈ 100 mcg once daily. Give on an empty stomach 30–60 minutes before breakfast, separated from calcium, iron, soy and proton-pump inhibitors by at least four hours. Recheck TSH and free T4 at 6 weeks, then titrate. ^[2][7]

Monitoring and long-term plan. Target TSH 1–5 mU/L with free T4 in the mid-to-upper range and symptom resolution; recheck every 4–6 months during growth and at any dose change. Screen coeliac serology (tTG-IgA + total IgA), and check HbA1c/glucose if any polyuria/polydipsia; reinforce annual thyroid surveillance is now part of her record. Plan a supervised withdrawal trial in stable older adolescence, as a minority of Hashimoto remits. ^[2][4]