A newborn boy is found to have markedly elevated serum TSH and low thyroxine on routine screening. Laboratory findings using a venous blood sample confirm the diagnosis. The patient was born at 39 weeks gestation to a 34-year-old healthy woman, who had been taking prenatal vitamins regularly. He has no symptoms or abnormal findings on clinical examination. Imaging studies reveal an ectopic thyroid gland just below the tongue. Which of the following is the most likely cause of this patient's otherwise normal physical examination despite abnormal thyroid laboratory findings?

Question

A newborn boy is found to have markedly elevated serum TSH and low thyroxine on routine screening.  Laboratory findings using a venous blood sample confirm the diagnosis.  The patient was born at 39 weeks gestation to a 34-year-old healthy woman, who had been taking prenatal vitamins regularly.  He has no symptoms or abnormal findings on clinical examination.  Imaging studies reveal an ectopic thyroid gland just below the tongue.  Which of the following is the most likely cause of this patient's otherwise normal physical examination despite abnormal thyroid laboratory findings?

Quizplus · Accepted Answer

11ec160b_3cb5_2b86_9249_b702aa824b87_MD0001_00 This infant has primary hypothyroidism, a congenital disorder that is most commonly caused by abnormal thyroid gland development (ie, agenesis, hypoplasia) or location (ectopy). During normal embryogenesis, the thyroid descends from the developing pharynx to its normal location anterior to the trachea. Abnormal migration results in an ectopic thyroid (eg, lingual, mediastinal), as seen in this patient. Most patients with thyroid dysgenesis have hypothyroidism, with routine newborn screening findings of increased TSH and decreased thyroxine (T4) levels. However, newborns are typically asymptomatic for the first weeks due to the presence of maternal T4, which crosses the placenta in small amounts during gestation. The amount of neonatal T4 is still low compared to that in unaffected patients but is sufficient to initially prevent clinical manifestations of hypothyroidism. As maternally derived neonatal T4 begins to wane in the first few weeks to months of life, classic symptoms of untreated hypothyroidism develop. Presenting signs include lethargy, hypotonia, constipation, and poor feeding. A hoarse cry, macroglossia, facial edema, and prolonged jaundice are also common. (Choice A) Ectopic thyroid tissue typically produces decreased, not increased, T4. (Choice B) Physiologically increased thyroid-binding globulin (TBG) levels in pregnancy result in increased total maternal T4 and triiodothyronine (T3) to maintain a euthyroid state. Because maternal levels of free T4 and T3 remain normal, the increase in maternal TBG does not affect the fetus. (Choice C) Transplacental transfer of thyroid-stimulating antibodies in mothers with Graves disease results in transient neonatal hyperthyroidism (neonatal Graves disease). Laboratory findings would show increased T4 and decreased TSH. (Choice E) TSH is a polypeptide hormone that does not cross the placenta. Educational objective: Congenital hypothyroidism, most commonly caused by abnormal thyroid gland development or location (ie, ectopy), is identified by elevated TSH and decreased thyroxine (T4) levels. Clinical manifestations of hypothyroidism are not present at birth due to the transplacental transfer of small amounts of maternal T4. __________ References: Congenital hypothyroidism. (http://www.ncbi.nlm.nih.gov/pubmed/32644339) Ectopic thyroid. (http://www.ncbi.nlm.nih.gov/pubmed/30969714)

A Newborn Boy Is Found to Have Markedly Elevated Serum