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]

SAQ 2 (10 marks) — Central congenital hypothyroidism and the missed screen

Stem: A six-week-old infant presents with failure to thrive, hypoglycaemia, and a micropenis. The newborn bloodspot TSH was normal. A venous free T4 is low and the TSH is inappropriately normal. Discuss the diagnosis, the immediate management priorities, and why the screen missed it. [5] [1]

Model answer

Diagnosis and why the screen missed it (3 marks). The diagnosis is central congenital hypothyroidism from panhypopituitarism — a low free T4 with an inappropriately normal or low TSH. The TSH-based bloodspot screen misses central disease because the TSH is not raised; the diagnosis rests on a low free T4 with a pituitary or midline context. The micropenis and hypoglycaemia point to coexistent growth-hormone and gonadotrophin deficiency. [5] [1]

Immediate management — hydrocortisone before levothyroxine (4 marks). Before starting levothyroxine, assess and replace cortisol, because starting levothyroxine first accelerates cortisol clearance and can precipitate an adrenal crisis in a child with unrecognised adrenal insufficiency. Give hydrocortisone replacement if adrenal insufficiency is confirmed or suspected, then start levothyroxine. In central disease the levothyroxine dose is titrated to the free T4 rather than the TSH, because the TSH is unreliable. [5] [1]

Full pituitary work-up (2 marks). Measure cortisol, growth hormone, prolactin, and the gonadal and posterior-pituitary axes, and arrange a magnetic resonance imaging of the pituitary and hypothalamus to define a structural lesion. Manage jointly with paediatric endocrinology and neurosurgery where a lesion is present. [5]

Prognosis and surveillance (1 mark). The child needs lifelong multidisciplinary endocrine care, with structured transition to adult endocrinology. The prognosis depends on the underlying pituitary cause and the adequacy of the hormone replacement, and the cognitive outcome is protected when all deficits are recognised and replaced early. [1]