Congenital hypothyroidism — branching viva

Opening: confirm and treat

The candidate opens by recognising that a bloodspot TSH of 68 mIU per litre requires venous confirmation and immediate treatment. Confirm with venous TSH and free T4; a high TSH with a low free T4 confirms primary congenital hypothyroidism. Start levothyroxine 10 to 15 mcg per kg per day the same day as a single daily oral dose on an empty stomach, without waiting for imaging or genetics. [1] [2]

The examiner probes the early signs: prolonged jaundice, a large posterior fontanelle, lethargy, constipation, and a low hoarse cry. The candidate explains that these are the non-specific early features that precede the classic untreated picture, and that screening exists precisely to catch the disease in this asymptomatic window. [3]

Branch 1: the dose, the timing, and the targets

The examiner asks why the dose is high and early. The candidate answers that the developing brain depends on thyroid hormone for myelination through the first thousand days, and that a high initial levothyroxine dose started within the first two weeks protects that window. The systematic-review evidence supports the 10 to 15 mcg per kg per day dose for neurodevelopmental benefit. [4]

The monitoring targets are explicit: free T4 normal within two weeks and TSH normal within four weeks, checked at two and four weeks, then dose-for-growth surveillance through infancy. The target is a free T4 in the upper half of the range and a TSH between 0.5 and 5.0 mIU per litre. A persistently high TSH on a correct dose usually means non-adherence or malabsorption, not dose inadequacy. [1] [2]

Branch 2: primary versus central, and the missed screen

The examiner branches to central disease. The candidate explains that primary disease carries a high TSH and the screen detects it, while central disease carries a low or inappropriately normal TSH and a TSH-only screen misses it. Any low free T4 with a pituitary or midline context — hypoglycaemia, micropenis, cholestasis, septo-optic dysplasia — needs venous tests regardless of the bloodspot. [3] [1]

The central-disease trap follows. The examiner asks what to give before levothyroxine in central disease with adrenal insufficiency. The candidate answers hydrocortisone, because starting levothyroxine first accelerates cortisol clearance and can precipitate an adrenal crisis. In central disease the dose is titrated to the free T4 rather than the TSH. [3]

Branch 3: the age-three permanence reassessment

The examiner moves to the long term. The candidate explains that the permanence of congenital hypothyroidism is provisional until the age-three reassessment, a supervised trial off therapy for four to six weeks with recheck of TSH and free T4. A rising TSH with a falling free T4 confirms permanent disease; a stable normal result confirms transient disease and treatment stops. The reassessment can be skipped when the gland is ectopic or absent or the starting dose was high. [2] [1]

Closing: the school-age child with an abnormal test

The examiner closes with the same child at age seven, with a recent TSH of 12 mIU per litre. The candidate explains that the commonest cause of an abnormal test in a treated child is a dose outgrown, re-weighs the child, recalculates the dose per kilogram, and adjusts. The candidate frames the prognosis honestly — near-normal outcomes with early full-dose treatment, with subtle residual deficits in severe or late-treated cases — and confirms that the surveillance continues to adult transition. [1]