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]

SAQ 2 (10)

A 14-year-old girl with type 1 diabetes attends her annual review. She feels well. TSH is 7.2 mU/L (reference 0.4–4.0), free T4 13 pmol/L (normal), anti-TPO positive. There is a small, smooth goitre. [5][4]

1. How do you classify this result, and what is the next investigation step? (3) [5]
2. Give the criteria that would lead you to start levothyroxine in subclinical hypothyroidism. (3) [5]
3. Explain why this patient warrants annual thyroid surveillance regardless of today's decision, and outline the counselling priorities. (4) [4]

Model answer

Classification and next step. This is subclinical hypothyroidism — TSH raised but free T4 normal. Up to a third of such results normalise spontaneously, so the next step is to re-test TSH and free T4 in 4–8 weeks before committing to lifelong treatment, confirming persistence rather than transient non-thyroidal drift. [5]

Criteria to treat. Start levothyroxine if there is a goitre, symptoms, a steadily rising TSH across serial tests, or anti-TPO positivity (all present to some degree here), or if TSH is persistently above 10 mU/L even when asymptomatic. The presence of a goitre, positive anti-TPO and T1DM together tips the balance toward treatment after confirming persistence. [5]

Surveillance and counselling. Type 1 diabetes carries anti-TPO positivity in 15–30% and overt thyroiditis in 5–10%; hypothyroidism destabilises glycaemia and growth, so annual TSH and free T4 is mandatory regardless of today's decision. Counsel that the condition is common, fully treatable, and reversible in terms of symptoms; address adherence (daily empty-stomach tablet, separate from iron/calcium/soy), the monitoring plan, the excellent prognosis with catch-up growth and preserved cognition, and — for an adolescent girl — pregnancy planning (levothyroxine needs rise 25–30%). [4]