A 5-month-old male infant is brought to the pediatric clinic by his parents for a well-chi… — USMLE Step 2 CK

A $5$ -month-old male infant is brought to the pediatric clinic by his parents for a well-child visit. The parents report that he is a "very quiet baby" who sleeps most of the day, but they are concerned because he frequently strains to pass hard, pellet-like stools. He is not yet rolling over and rarely coos. He was born at $39$ weeks gestation via uncomplicated vaginal delivery. The state's newborn screening program primarily measures thyroid-stimulating hormone (TSH) levels, which were reported as normal. On physical examination, his length is at the $5$ th percentile and weight is at the $25$ th percentile. His heart rate is $90$ /min and temperature is $36.4^\circ\text{C}$ ( $97.5^\circ\text{F}$ ). He exhibits generalized hypotonia and a stretched penile length of $1.5 \text{ cm}$ (normal $> 2.5 \text{ cm}$ ). Laboratory evaluation reveals:

Serum thyroid-stimulating hormone (TSH): $2.1 \mu\text{U/mL}$ (normal: $0.5$ – $5.0 \mu\text{U/mL}$ )
Serum free thyroxine (T4): $0.4 \text{ ng/dL}$ (normal: $0.9$ – $2.2 \text{ ng/dL}$ )
Serum glucose: $52 \text{ mg/dL}$

Which of the following is the most likely cause of this infant's condition?

A
Defective embryologic development of the thyroid gland
B
Decreased production of thyroxine-binding globulin
Deficiency of hypothalamic or pituitary hormone productionAnswer
D
Peripheral tissue resistance to thyroid hormone
E
Normal physiologic development for age

Answer

Deficiency of hypothalamic or pituitary hormone production

This infant is presenting with classic signs of hypothyroidism, including lethargy, severe constipation, hypotonia, and poor growth. However, the laboratory profile shows a low free T4 with a normal TSH. This lack of an expected TSH surge in response to low thyroxine indicates a central defect (secondary or tertiary hypothyroidism). Furthermore, the presence of a micropenis (indicating luteinizing hormone/follicle-stimulating hormone or growth hormone deficiency) and borderline hypoglycemia (indicating growth hormone or adrenocorticotropic hormone deficiency) strongly points toward panhypopituitarism. Newborn screening programs in many states rely solely on TSH levels to detect primary congenital hypothyroidism and will therefore miss central hypothyroidism.

Step-by-Step Solution

Analyze the clinical presentation.

The infant has lethargy, constipation, hypotonia, delayed milestones, micropenis, and borderline hypoglycemia.

Identifying the clinical syndrome guides the interpretation of the laboratory values.

Interpret the thyroid function tests.

Free T4 is clearly low (indicating hypothyroidism), but the TSH is inappropriately normal.

A normal TSH in the face of low free T4 localizes the defect to the hypothalamus or pituitary gland (central hypothyroidism), rather than the thyroid gland itself.

Synthesize the endocrine findings.

The presence of central hypothyroidism alongside micropenis (suggesting gonadotropin/GH deficiency) and borderline hypoglycemia (suggesting ACTH/GH deficiency) points to panhypopituitarism.

Recognizing that multiple pituitary axes are affected confirms the hypothalamic-pituitary origin of the infant's disease.

Key Concept

Central hypothyroidism is characterized by a low free T4 with an inappropriately normal or low TSH. In neonates and infants, it is often part of panhypopituitarism (indicated by concurrent hypoglycemia and micropenis) and is frequently missed by newborn screens that only measure TSH.

Estimated Time:1m 30s

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