A breastfeeding infant, screened for congenital hypothyroidism, is found to have low levels of thyroxine (T4) and high levels of thyroid-stimulating hormone (TSH). What is the best explanation for this finding?

Choice A rationale While it’s true that TSH levels can influence thyroxine levels, this statement is misleading. High TSH levels do not cause low thyroxine levels. Instead, the pituitary gland produces more TSH when thyroxine levels are low in an attempt to stimulate the thyroid gland to produce more thyroxine. Choice B rationale Thyroxine levels in breastfeeding infants should be within the normal range. High thyroxine levels are not typical and could indicate a problem with the infant’s thyroid gland. Choice C rationale This statement is incorrect. The thyroid gland begins producing thyroxine before birth, and levels should be within the normal range shortly after birth. Choice D rationale This is the correct explanation. In congenital hypothyroidism, the thyroid gland does not produce enough thyroxine, leading to high levels of TSH. The pituitary gland produces more TSH in an attempt to stimulate the thyroid gland to produce more thyroxine.