First Aid for the USMLE Step 1 Casebook – A newborn boy with apparent birth defects

A newborn boy is found to have an increased head circumference, macroglossia, an umbilical hernia, lethargy, and a hoarse cry. The mother has no significant past medical history, takes no medications, and received adequate prenatal care. His serum thyroxine (T4) concentration is ~50% that of normal infants.

>>>What is the most likely diagnosis?

Congenital hypothyroidism (aka cretinism).

>>>Lack of adequate treatment of this condition is likely to result in what long-term consequence?

Congenital hypothyroidism is the most common treatable cause of mental retardation. Children with the condition were once popularly known as cretins (from the French word Chrétien or Christ-like), as they were so profoundly mentally retarded as to be considered incapable of sinning.

>>>What are the most common causes of this condition?

Eighty-five percent are sporadic and 15% are inherited. The most common causes are thyroid dysgenesis (ectopy, hypoplasia, or complete agenesis); disordered thyroid hormone synthesis, secretion, or transport; and iodine deficiency. The condition is also more common in places where goiter is endemic.

>>>How might thyroid disease in the mother affect the developing fetus?

In mothers with autoimmune hypothyroidism, thyroid-stimulating hormone-receptor blocking antibodies can cross the placenta and affect the fetus’ thyroid function. This “transient congenital hypothyroidism” should resolve within 1–3 months, when maternal antibodies are cleared from the infant’s system. Hyperthyroid women are more likely to have difficulties conceiving and have higher rates of miscarriage. Hyperthyroidism must be controlled in pregnancy, as the risks of miscarriage and birth defects are much higher without treatment.

>>>Which thyroid medications are safe in pregnancy?

Thyroid hormone replacements (such as levothyroxine) for hypothyroidism are safe in pregnancy, given that the medication is appropriately prescribed and consumed. Antithyroid drugs given to hyperthyroid mothers can cross the placenta. Propylthiouracil (PTU) is the drug of choice for treating hyperthyroidism in pregnancy, as its transplacental effects on the fetus are less severe. PTU is cleared from the infant’s system within a few days after birth. Methimazole readily crosses the placenta and should be avoided in pregnancy, as should medications with large quantities of iodine, such as amiodarone for cardiac arrhythmia.


This practice case study is representative of the studies available in the First Aid™ Cases for the USMLE Step 1.  First Aid™ Cases for the USMLE Step 1 features 400 well-illustrated cases to help you relate basic science concepts to clinical situations. Each case includes drawings or clinical images with Q&As that reinforce key concepts. Get more Step 1 study help at


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