What is the adequate level of growth hormone (GH) for a 14-year-old?

Adequate Growth Hormone Levels for a 14-Year-Old

For a 14-year-old, adequate growth hormone (GH) levels are not defined by a single numerical value but rather by functional assessment: normal GH secretion is demonstrated when GH suppresses below 1 μg/L after an oral glucose load in adults, though this cutoff is problematic in adolescents where approximately 30% of normally growing tall children fail to suppress below this threshold. 1, 2

Understanding GH Assessment in Adolescents

The evaluation of "adequate" GH in a 14-year-old differs fundamentally from adults because:

• Pubertal status matters more than absolute GH levels. At age 14, most adolescents are in mid-to-late puberty (Tanner stages 2-4), where GH dynamics are sex and pubertal stage-specific, with the highest GH nadirs after glucose load occurring in mid-puberty (Tanner stage 2-3), particularly in girls (mean ± 2 SD: 0.22 μg/L ± 0.03-1.57 in girls vs 0.21 μg/L ± 0.09-0.48 in boys). 1, 2

• Growth velocity is the most clinically relevant marker. Rather than focusing on serum GH levels, adequacy is best judged by whether the adolescent maintains a height velocity above the 25th percentile for age and sex. 1

Clinical Indicators of Adequate GH Function

The following parameters indicate adequate GH function in a 14-year-old:

• Normal growth velocity: Height velocity at or above the 25th percentile for age and sex indicates sufficient GH action. 1

• Appropriate pubertal progression: Assessment using Tanner staging in patients older than 10 years should show age-appropriate sexual development, as pubertal delay can indicate GH-related issues. 1

• IGF-1 levels within age-matched reference ranges: Serum IGF-1 should fall within local Tanner stage-matched, sex-matched, and age-matched normal ranges, though this must account for inter-assay variability. 1, 2

• Height tracking along growth curve: Height should remain above the 3rd percentile (or height SDS above -1.88) without progressive decline. 1

When GH Insufficiency Should Be Suspected

Consider GH insufficiency if the 14-year-old demonstrates:

• Height below the 3rd percentile for age and sex AND height velocity below the 25th percentile persisting beyond 6 months. 1

• Delayed puberty: boys with testicular volume less than 4 mL at age 14 years or girls with breast stage less than B2 at age 13.5 years warrant endocrine evaluation. 1

• Low serum IGF-1 levels for age, sex, and pubertal stage (though this can be confounded by hypothyroidism, malnutrition, severe infection, or poorly controlled diabetes). 1, 2

Important Caveats for Adolescent GH Testing

Several pitfalls complicate GH assessment at age 14:

• Standard adult GH suppression testing is unreliable in adolescents. Approximately 30% of children with tall stature fail to suppress GH below 1 μg/L after glucose load despite being completely normal. 1, 2

• Provocative GH testing (stimulation tests) has limited value. Many children previously diagnosed with "GH insufficiency" based on peak GH responses of 7.7-13.6 mU/L (approximately 3.5-6.2 μg/L) during childhood showed completely normal GH secretion (peak GH 10.4-100 mU/L) when retested at completion of linear growth. 3

• IGF-1 interpretation requires proper reference ranges. Values can be falsely low with concurrent hypothyroidism, malnutrition, or severe infection, and falsely elevated with poorly controlled diabetes, hepatic/renal failure, or in girls taking oral estrogens. 1, 2

Practical Clinical Approach

For a 14-year-old with growth concerns, prioritize these assessments:

1. Plot growth parameters: Current height, height velocity over the past 6-12 months, and parental target height. 1

2. Assess pubertal development: Document Tanner stage to contextualize GH dynamics. 1

3. Obtain bone age radiograph: Left wrist X-ray to assess growth potential and whether epiphyses remain open. 1

4. Measure IGF-1 and thyroid function: Serum IGF-1 (age/sex/Tanner-matched reference ranges) and TSH/free T4 to exclude hypothyroidism. 1, 2

5. Consider GH stimulation testing only if: Growth velocity is persistently low AND IGF-1 is low for pubertal stage, recognizing that many "insufficient" results in adolescence normalize after puberty. 3

The bottom line: "Adequate" GH at age 14 is best defined functionally by normal growth velocity (≥25th percentile), appropriate pubertal progression, and age-appropriate IGF-1 levels, rather than by arbitrary GH cutoffs that perform poorly in this age group.