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]}