Why should cortisol be evaluated before initiating growth hormone therapy?

Why Cortisol Must Be Evaluated Before Starting Growth Hormone Therapy

Cortisol assessment is mandatory before initiating growth hormone (GH) therapy because GH can unmask or precipitate adrenal insufficiency by accelerating cortisol metabolism, potentially leading to life-threatening adrenal crisis.

The Critical Mechanism: GH's Effect on Cortisol Metabolism

Growth hormone directly inhibits 11β-hydroxysteroid dehydrogenase type 1 (11β-HSD1), the enzyme responsible for converting inactive cortisone to active cortisol 1. This biochemical interaction has several clinically significant consequences:

• GH accelerates peripheral cortisol clearance, reducing cortisol availability by approximately 20-40% 2, 3
• GH decreases the ratio of cortisol metabolites to cortisone metabolites in urine, indicating enhanced cortisol breakdown 3, 4
• GH suppresses endogenous cortisol production rates by approximately 40-45% in healthy individuals 5

Why This Matters Clinically

Unmasking Hidden Adrenal Insufficiency

In patients with hypopituitarism or multiple pituitary hormone deficiencies, pre-existing but compensated ACTH deficiency may become clinically apparent only after GH initiation 6, 1. The enhanced cortisol metabolism induced by GH can precipitate frank adrenal crisis in patients with marginal adrenal reserve.

The Prader-Willi Syndrome Experience

The PWS guidelines provide the most explicit warning: early-morning ACTH and cortisol must be evaluated when the child is well, and consideration should be given to prophylactic hydrocortisone during critical illness 7. This recommendation stems from recognition that centrally mediated adrenal insufficiency may be an underrecognized contributor to premature deaths in PWS patients receiving GH.

Evidence from Multiple Populations

Research demonstrates that:

• Asymptomatic ACTH deficiency occurs in approximately 40% of patients with isolated GH deficiency despite normal thyroid function 8
• GH deficiency itself may mask underlying adrenal insufficiency because the GH-deficient state enhances cortisol availability through increased 11β-HSD1 activity 1, 2
• Glucocorticoid levels are required for optimal GH secretion, creating a bidirectional relationship 9, 10

Practical Testing Algorithm

Before GH Initiation:

1. Measure 8 AM cortisol and ACTH levels (after age 6-12 months when diurnal rhythm is established) 11

2. Interpret results:

   • Cortisol <5 μg/dL (138 nmol/L): Likely adrenal insufficiency—proceed to stimulation testing
   • Cortisol 5-10 μg/dL with ACTH >100 pg/mL: Indeterminate—perform ACTH stimulation test 12
   • Cortisol >18 μg/dL (500 nmol/L): Excludes ACTH insufficiency 11

3. ACTH stimulation testing when indicated:

   • Peak cortisol >18 μg/dL (500 nmol/L) excludes adrenal insufficiency 11
   • Consider insulin tolerance test in specialized settings for definitive assessment 11

After GH Initiation:

The hypothalamic-pituitary-adrenal axis should be reassessed during GH therapy in patients not previously found to be ACTH-deficient 6. This is because:

• GH may unmask latent adrenal insufficiency within weeks to months
• Patients may require glucocorticoid replacement that wasn't needed before GH

Management Implications

If Adrenal Insufficiency is Confirmed:

1. Initiate hydrocortisone replacement BEFORE starting GH 7, 6

   • Typical dosing: 15-20 mg/day in adults (divided doses)
   • Pediatric dosing: 8-10 mg/m²/day in children

2. Provide comprehensive patient education on:

   • Stress dosing protocols
   • Emergency injectable hydrocortisone
   • Medical alert identification 13

3. Monitor during GH therapy:

   • Reassess if symptoms of adrenal insufficiency develop
   • Consider repeat testing if clinical suspicion arises

Special Populations Requiring Extra Vigilance:

• Patients with multiple pituitary hormone deficiencies (highest risk)
• Those receiving cortisone acetate (requires conversion to cortisol, which GH inhibits) 4
• Patients with Prader-Willi syndrome (documented increased mortality risk) 7
• Children with chronic kidney disease (multiple metabolic stressors) 14

The Bottom Line

Failure to assess cortisol status before GH therapy can result in preventable adrenal crises. The interaction between GH and cortisol metabolism is not theoretical—it is a well-documented biochemical phenomenon with potentially fatal clinical consequences. The small investment in pre-treatment cortisol assessment protects against a life-threatening complication that becomes more likely once GH therapy begins.