Growth Hormone Deficiency
The most likely diagnosis is Growth Hormone Deficiency (Option C), based on the severely low IGF-1 level (18 ng/mL, dramatically below the normal range of 67-405 ng/mL), 3-year delayed bone age, and proportionate short stature in this 9-year-old boy. 1, 2
Diagnostic Reasoning
The severely low IGF-1 is the critical discriminator in this case:
IGF-1 of 18 ng/mL represents profound deficiency - this level is less than 30% of the lower limit of normal (67 ng/mL) for age, indicating significant GH axis dysfunction rather than a normal variant 1, 2
The 3-year bone age delay (6 years at chronological age 9) is characteristic of GHD and distinguishes this from familial short stature, which presents with normal bone age 3, 1, 2
Proportionate short stature (height 120 cm at age 9, approximately -3 SD) suggests an endocrine cause rather than skeletal dysplasia 1, 2
Normal other pituitary hormones (TSH 3.2, FSH 1.4, LH 0.1) indicate isolated GHD rather than multiple pituitary hormone deficiency 1
Why Other Options Are Incorrect
Failure to Thrive (Option A) is excluded because this term applies to infants and young children with inadequate weight gain and poor caloric intake, typically accompanied by feeding difficulties - this 9-year-old is asymptomatic with normal physical examination 3, 2
Familial Short Stature (Option B) is unlikely because:
- Familial short stature presents with normal bone age and normal growth velocity 3, 2
- IGF-1 levels are typically normal in familial short stature 2
- While the mother has short stature, the severely delayed bone age and profoundly low IGF-1 indicate pathology beyond genetic potential 3, 1
Clinical Implications
This patient requires GH stimulation testing to confirm GHD, as the severely low IGF-1 and delayed bone age provide strong evidence but formal diagnosis requires demonstration of inadequate GH secretion 1, 2
If GHD is confirmed, recombinant human GH therapy at 0.045-0.05 mg/kg/day is indicated, with the 3-year delayed bone age suggesting substantial remaining growth potential for catch-up growth 1
Common Pitfalls to Avoid
Do not assume familial short stature based solely on maternal height - the severely low IGF-1 and delayed bone age indicate pathologic rather than genetic short stature 3, 2
Do not overlook the magnitude of IGF-1 deficiency - levels this low (18 ng/mL vs normal 67-405 ng/mL) are not seen in normal variants and demand investigation for GH axis dysfunction 1, 2
Monitor bone age every 6 months during treatment to optimize final adult height and prevent premature epiphyseal closure 4