In a 1‑month‑old infant with poor weight gain after ruling out inborn errors of metabolism and congenital hypothyroidism, what is the most likely endocrine cause and what initial endocrine work‑up should be performed?

Growth Hormone Deficiency (GHD) with Possible Multiple Pituitary Hormone Deficiencies

In a 1-month-old infant with poor weight gain after ruling out inborn errors of metabolism and congenital hypothyroidism, the most likely endocrine cause is congenital growth hormone deficiency (GHD), often presenting as part of multiple pituitary hormone deficiencies (MPHD), and the initial work-up should include a random serum GH level, IGFBP-3, cortisol, and electrolytes to assess for associated ACTH deficiency. 1

Clinical Reasoning

At 1 month of age, neonatal GHD typically manifests with:

• Hypoglycemia (especially if ACTH deficiency coexists)
• Jaundice (from impaired liver function in MPHD with central hypothyroidism, GH, and ACTH deficiencies)
• Micropenis in males (if isolated GHD or associated with GnRH deficiency)
• Poor weight gain/failure to thrive 1

Importantly, neonates with GHD usually have normal birth weight for gestational age, distinguishing them from primary IGF-I deficiency which presents with intrauterine growth retardation. Growth velocity decline typically becomes more apparent between 6-12 months when the GH-IGF-I axis becomes the primary driver of linear growth 1.

Initial Endocrine Work-Up

Primary Diagnostic Tests

Random serum GH level:

• A random GH level ≤5 μg/L (5 ng/mL) in the first week of life plus other pituitary hormone deficiencies is sufficient to diagnose GHD per Pediatric Endocrine Society guidelines 1
• A cutoff of <4.5 ng/mL achieves 100% sensitivity and 83% specificity for congenital GHD during the first 28 days of life 1
• GH provocation testing is labor-intensive and potentially hazardous in infants, making random GH levels the preferred diagnostic approach 1

IGFBP-3 level:

• More useful than IGF-I in children up to 3 years of age 1
• IGF-I has limited utility within the first 18 months due to confounding factors (weight gain, adiposity, feeding type, maternal smoking) 1

Assessment for Associated Hormone Deficiencies

Cortisol and ACTH evaluation:

• Critical because ACTH deficiency commonly coexists with GHD and causes hypoglycemia 1
• Check morning cortisol and consider ACTH stimulation if borderline

Serum electrolytes:

• To evaluate for potential adrenal insufficiency
• Look for hyponatremia, hyperkalemia

Thyroid function tests:

• Since congenital hypothyroidism is already ruled out, but central hypothyroidism can be part of MPHD 1
• Free T4 and TSH (TSH may be inappropriately normal or low in central hypothyroidism)

Additional Considerations

Physical examination specifics:

• Examine for micropenis in males (stretched penile length <2.5 cm suggests GnRH or GH deficiency) 1
• Look for midline facial defects suggesting septo-optic dysplasia
• Assess for nystagmus or visual abnormalities (optic nerve hypoplasia)

MRI brain/pituitary:

• Should be obtained once biochemical diagnosis is established
• Look for pituitary stalk abnormalities, ectopic posterior pituitary (EPP), anterior pituitary hypoplasia, septo-optic dysplasia, or holoprosencephaly 1

Common Pitfalls

• Do not rely on IGF-I levels in this age group—they are unreliable and affected by nutritional status 1
• Do not delay evaluation for hypoglycemia—this can be life-threatening, especially with concurrent ACTH deficiency 1
• Do not assume normal newborn screen rules out all endocrine causes—GHD and MPHD are not detected by standard newborn screening 1
• Remember that jaundice in a neonate with poor weight gain may signal MPHD rather than simple feeding issues 1

Underlying Etiologies to Consider

Congenital malformations causing neonatal GHD include:

• Septo-optic dysplasia with optic nerve hypoplasia
• Holoprosencephaly
• Pituitary stalk abnormalities
• Ectopic posterior pituitary with anterior pituitary hypoplasia 1

These structural abnormalities explain why MPHD is common in congenital GHD presentations.