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Difficulty: HardPediatric Endocrine Disorders

A 99-month-old boy is brought to the clinic for a routine evaluation. His parents are concerned because he is not yet sitting without support and seems less active than his older siblings were at this age. He was born at 4040 weeks gestation following an uncomplicated pregnancy. The state newborn screening, which utilizes a primary thyroid-stimulating hormone (TSH) assay, was reported as normal. Over the last six months, his length has fallen from the 5050 th to the 55 th percentile, and his weight has fallen from the 4040 th to the 1010 th percentile. On physical examination, the infant has generalized hypotonia, a prominent forehead, and a remarkably small phallus.

Laboratory testing reveals the following:

Laboratory TestResultReference Range
Serum free thyroxine (T4)0.50.5 ng/dL0.90.9-2.32.3 ng/dL
Serum thyroid-stimulating hormone (TSH)2.22.2 mIU/L0.50.5-5.05.0 mIU/L
Serum glucose5555 mg/dL7070-100100 mg/dL

Which of the following is the most likely underlying cause of this patient's clinical and laboratory findings?

  1. Deficient secretion of anterior pituitary hormonesAnswer
  2. B
    Failure of embryonic thyroid gland development
  3. C
    Autoimmune destruction of thyroid follicles
  4. D
    Peripheral end-organ resistance to thyroid hormone
  5. E
    Benign familial pattern of delayed physical maturation

Answer

Deficient secretion of anterior pituitary hormones
The correct answer identifies central (secondary) hypothyroidism as part of a broader congenital hypopituitarism syndrome. The patient has a low free T4 level with an inappropriately normal TSH, indicating a central defect rather than a primary thyroid gland issue. The presence of a remarkably small phallus (micropenis) and hypoglycemia strongly suggests concomitant growth hormone (GH) and gonadotropin deficiency, which is classic for congenital hypopituitarism. Notably, newborn screening programs that rely primarily on TSH assays often miss central hypothyroidism because TSH levels are not elevated.

Step-by-Step Solution

1
Interpret the thyroid function tests.
The free T4 is abnormally low, but the TSH is within the normal range.
A low free T4 should normally trigger a robust TSH elevation via negative feedback. An inappropriately normal TSH in the setting of low T4 indicates a central (pituitary or hypothalamic) defect, known as central hypothyroidism.
2
Evaluate the clinical signs and other laboratory findings in the context of a central endocrine defect.
The patient has hypoglycemia, growth failure, and a small phallus (micropenis).
These findings point to additional anterior pituitary hormone deficiencies. Growth hormone (GH) and adrenocorticotropic hormone (ACTH) deficiencies cause hypoglycemia and growth failure. Gonadotropin (LH/FSH) and GH deficiencies during fetal development result in a micropenis.
3
Synthesize the findings to determine the underlying etiology.
The combination of central hypothyroidism with suspected GH, ACTH, and gonadotropin deficiencies indicates panhypopituitarism.
Congenital hypopituitarism perfectly explains the broad spectrum of metabolic and developmental abnormalities seen in this infant.

Key Concept

Congenital Hypopituitarism and Central Hypothyroidism
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Topics

Pediatric Endocrine Disorders

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