A neonate is found to have increased TSH and low thyroxine (T4) on routine screening. He was born by vaginal delivery to a 32-year-old woman at 39 weeks gestation. Apgar scores were 8 and 9 at 1 and 5 minutes, respectively. Physical examination reveals an enlarged thyroid gland in the normal position. Neck ultrasound confirms thyroid enlargement. Which of the following is the most likely cause of this patient's findings?

Question

A neonate is found to have increased TSH and low thyroxine (T4) on routine screening.  He was born by vaginal delivery to a 32-year-old woman at 39 weeks gestation.  Apgar scores were 8 and 9 at 1 and 5 minutes, respectively.  Physical examination reveals an enlarged thyroid gland in the normal position.  Neck ultrasound confirms thyroid enlargement.  Which of the following is the most likely cause of this patient's findings?

Quizplus · Accepted Answer

11ec160b_3cb5_a0b7_9249_b9d3dd7b381f_MD0001_00 This neonate has a goiter, or an enlarged thyroid gland, which is typically due to thyroid hormone dysregulation. The most commonly associated imbalance is hypothyroidism, as seen in this patient with elevated TSH and decreased thyroxine (T4). One cause of neonatal hypothyroidism with a goiter is the transplacental passage of maternal antithyroid medications (eg, propylthiouracil, methimazole). Propylthiouracil is used to treat maternal hyperthyroidism (eg, Graves disease) because it inhibits thyroid peroxidase, thereby limiting oxidation and incorporation of iodine into thyroid hormones; this medication also decreases conversion of peripheral T4 to triiodothyronine (T3). Propylthiouracil crosses the placenta, inhibiting production of T4. Positive feedback caused by low T4 leads to increased TSH, which stimulates the thyroid and causes diffuse gland enlargement. TSH and T4 levels typically normalize after 1-2 weeks due to excretion of the maternally derived antithyroid medication. After elimination of propylthiouracil, neonates with mothers who have Graves disease may then develop transient hyperthyroidism (↓ TSH, ↑ T4) due to transplacental transfer of TSH receptor-stimulating antibodies (Choice E), which can persist in the neonatal circulation for up to 3 months. (Choice A) Levothyroxine (synthetic T4), like other thyroid hormones, does not readily cross the placenta. In addition, this medication treats, not causes, hypothyroidism. (Choice B) Beta blockers (eg, propranolol) treat symptoms of hyperthyroidism attributable to increased beta-adrenergic activity (eg, palpitations, tachycardia). Although propranolol also decreases conversion of T4 to T3, this inhibitory effect is not strong enough to cause overt hypothyroidism and goiter at normal doses. (Choice D) Thyroglobulin is a protein produced by the thyroid follicular cells and is involved in the storage and production of thyroid hormones. Thyroglobulin does not cross the placenta or cause hypothyroidism. Educational objective: Congenital goiter in the setting of hypothyroidism (ie, ↑ TSH, ↓ thyroxine) can be caused by transplacental passage of maternal antithyroid medications (eg, propylthiouracil), which inhibit thyroid peroxidase and are used to treat maternal hyperthyroidism. __________ References: Congenital hypothyroidism. (http://www.ncbi.nlm.nih.gov/pubmed/32644339)

A Neonate Is Found to Have Increased TSH and Low