A 6-month-old boy is brought to the office for a well-child visit. He is babbling, sitting with support, and rolling over. The patient is exclusively breastfed and has not yet started solid foods. The parents note that he has been straining while passing stool, which has been hard in consistency, over the last few weeks. The patient was diagnosed with congenital hypothyroidism based on newborn screening results, and levothyroxine was initiated at age 2 weeks. He also takes daily vitamin D and iron supplements. The patient has no other chronic medical conditions. Vital signs are normal for age. Weight gain is appropriate for age, but length has decreased from the 60th to the 40th percentile since the last visit. Physical examination is unremarkable. Thyroid function testing at age 4 months was normal. Results of routine follow-up laboratory testing are as follows: Which of the following is the most likely mechanism for this patient's abnormal thyroid function tests?

Question

A 6-month-old boy is brought to the office for a well-child visit.  He is babbling, sitting with support, and rolling over.  The patient is exclusively breastfed and has not yet started solid foods.  The parents note that he has been straining while passing stool, which has been hard in consistency, over the last few weeks.  The patient was diagnosed with congenital hypothyroidism based on newborn screening results, and levothyroxine was initiated at age 2 weeks.  He also takes daily vitamin D and iron supplements.  The patient has no other chronic medical conditions.  Vital signs are normal for age.  Weight gain is appropriate for age, but length has decreased from the 60th to the 40th percentile since the last visit.  Physical examination is unremarkable.  Thyroid function testing at age 4 months was normal.  Results of routine follow-up laboratory testing are as follows:  Which of the following is the most likely mechanism for this patient's abnormal thyroid function tests?

Quizplus · Accepted Answer

11ec160b_3bbe_6f95_9249_4d5977501893_MD0001_00 This patient with treated congenital hypothyroidism has elevated TSH and low free thyroxine (T4), findings concerning for ineffective therapy with synthetic thyroid hormone (ie, levothyroxine). The most common cause of inadequate thyroid hormone levels in infants with treated congenital hypothyroidism is rapid growth, which increases the physiologic need for thyroid hormone. Another common reason for ineffective therapy is decreased absorption of levothyroxine. Coadministration with various foods (eg, soy products) or drugs (eg, iron, calcium, antacids) can lead to poor intestinal absorption of levothyroxine. In this case, iron binding to levothyroxine in the gastrointestinal tract caused circulating levothyroxine levels to decrease, resulting in symptoms of hypothyroidism (eg, constipation, poor linear growth) and increased TSH production. Because this patient's thyroid function tests at age 4 months were normal, iron supplementation was likely started in the past 2 months to treat the physiologic waning of iron stores that occurs at this age. (Choice B) Thyroxine-binding globulin (TBG) deficiency is characterized by low total T4 but normal TSH and free T4 levels. Patients do not require treatment because they are asymptomatic and euthyroid. (Choice C) Excessive thyroid hormone supplementation would result in inhibition of TSH production and elevated T4 levels with signs of hyperthyroidism (eg, tachycardia, irritability, increased bowel movements). (Choice D) Deficiency of iodine, which is required for thyroid hormone synthesis, rarely causes hypothyroidism in resource-rich countries due to dietary iodine found in salt. Breast milk also provides adequate iodine for infants, making this diagnosis unlikely. (Choice E) Thyroid hormone resistance is caused by thyroid hormone receptor mutations and results in elevated TSH and T4 levels due to impaired feedback inhibition. Signs of hypothyroidism or hyperthyroidism can occur due to varying tissue sensitivity to thyroid hormones. Educational objective: Coadministration of levothyroxine with various foods (eg, soy products) and certain medications (eg, iron, calcium, antacids) can lead to poor intestinal absorption of levothyroxine and symptoms of hypothyroidism while on therapy. __________ References: A review of the pharmacokinetics of levothyroxine for the treatment of hypothyroidism. (http://www.ncbi.nlm.nih.gov/pubmed/30349610)

A 6-Month-Old Boy Is Brought to the Office for a Well-Child