A 1-week-old newborn is seen for abnormal thyroid function tests that were performed as part of routine screening. Serum thyroxine (T4) was low and serum TSH was normal. The patient was born by spontaneous vaginal delivery at 40 weeks of gestation to a 28-year-old primigravida. During the pregnancy, the mother took acetaminophen intermittently for low back pain. Examination of the newborn shows no abnormal findings. Repeat thyroid function test results are as follows: Which of the following is the most likely cause of the patient's abnormal thyroid function tests?

Question

A 1-week-old newborn is seen for abnormal thyroid function tests that were performed as part of routine screening.  Serum thyroxine (T4) was low and serum TSH was normal.  The patient was born by spontaneous vaginal delivery at 40 weeks of gestation to a 28-year-old primigravida.  During the pregnancy, the mother took acetaminophen intermittently for low back pain.  Examination of the newborn shows no abnormal findings.  Repeat thyroid function test results are as follows:  Which of the following is the most likely cause of the patient's abnormal thyroid function tests?

Quizplus · Accepted Answer

11ec160b_3cb4_4124_9249_a5c562c1f4ec_MD0001_00 This neonate with a low total thyroxine (T4) but normal free T4 and normal TSH likely has a deficiency of thyroxine-binding globulin (TBG). Congenital TBG deficiency is a benign, X-linked disorder usually identified on newborn screening. In general, the majority of circulating T4 is bound to TBG, which then serves as a storage pool to replenish the biologically active free T4 that is continuously cleared by the kidneys. Therefore, TBG helps maintain a constant free T4 level. When TBG is deficient, less T4 is bound and a greater proportion is allowed to freely circulate. Total T4 is low due to the decrease in bound T4 fraction. However, since physiologic feedback is intact, TSH and free T4 levels remain normal. Because the levels of active (free) thyroid hormone are normal, patients are clinically euthyroid and asymptomatic, so no treatment is required. (Choice A) Certain placentally transferred maternal medications (eg, propylthiouracil) decrease conversion of T4 to triiodothyronine (T3), causing transient neonatal hypothyroidism (ie, low T4, high TSH). Acetaminophen does not interfere with thyroid hormone synthesis. (Choice C) Because only a small percentage of T4 is bound to albumin, hypoalbuminia does not significantly affect thyroid hormone levels. In addition, this patient has no risk factors for hypoalbuminemia (eg, malnutrition, sepsis, gastrointestinal/renal loss). (Choice D) Hypopituitarism causes central hypothyroidism, which is characterized by low TSH, total T4, and free T4. Other midline defects (eg, cleft lip) or hormonal imbalances (eg, hypoglycemia due to ACTH deficiency) are often present. (Choice E) Thyroid dysgenesis (eg, ectopy, hypoplasia) is the most common cause of congenital hypothyroidism, which is usually asymptomatic at birth. In contrast to this case, laboratory findings include high TSH and low total and free T4. Educational objective: Thyroxine-binding globulin deficiency is a benign disorder characterized by low total thyroxine (T4) and normal free T4 and TSH levels. Patients are euthyroid and do not require treatment. __________ References: Inherited defects in thyroxine binding globulins. (http://www.ncbi.nlm.nih.gov/pubmed/26522458)

A 1-Week-Old Newborn Is Seen for Abnormal Thyroid Function Tests