Congenital hypothyroidism — formative SAQs

SAQ 1 (10 marks) — The abnormal newborn bloodspot TSH

Stem: A four-day-old term infant is referred after the newborn bloodspot screen returns a TSH of 68 mIU per litre. The baby is clinically well, breastfeeds normally, and is jaundiced to the chest. The mother took carbimazole for Graves disease through pregnancy. Outline your immediate and long-term management. ^{[2] [3]}

Model answer

Confirm the diagnosis (2 marks). Take a venous sample for TSH and free T4 before starting treatment. A high TSH with a low free T4 confirms primary congenital hypothyroidism; a high TSH with a normal free T4 indicates subclinical or compensated disease. Do not repeat the bloodspot — the venous test is the diagnostic confirmation. ^{[1] [3]}

Start levothyroxine at full dose the same day (3 marks). Levothyroxine 10 to 15 mcg per kg per day as a single daily oral dose on an empty stomach. Crush the tablet and give via an open nipple with a small volume of milk or water. Do not wait for thyroid imaging or genetic results — the chemical diagnosis is sufficient. ^{[2] [4]}

Set and check the monitoring targets (2 marks). Free T4 should normalise within two weeks and TSH within four weeks. Check venous thyroid function at two and four weeks, then every one to two months through the first year. The target is a free T4 in the upper half of the age-appropriate range and a TSH between 0.5 and 5.0 mIU per litre. ^{[1] [3]}

Interpret the maternal context (2 marks). Maternal carbimazole and transplacental thyroid-blocking antibodies can cause transient neonatal hypothyroidism, so this infant's disease may be transient. Treat as congenital hypothyroidism regardless, and reassess permanence at age three with a supervised trial off therapy for four to six weeks, unless an ectopic or absent gland or a high starting dose declares the disease permanent. ^{[5] [2]}

Surveillance and counselling (1 mark). Re-weigh and re-dose at every visit, because the commonest cause of an abnormal test is a dose outgrown. Counsel the family that outcomes are near-normal with early full-dose treatment, and build developmental and audiological surveillance into the plan. ^[1]