Endocrine Manifestations of McCune-Albright Syndrome
McCune-Albright syndrome presents with multiple hyperfunctioning endocrinopathies, most commonly precocious puberty (especially in girls), followed by growth hormone excess, hyperthyroidism, hypercortisolism, hypophosphatemic rickets, and rarely hyperprolactinemia or hyperparathyroidism. 1, 2
Primary Endocrine Manifestations
Precocious Puberty
- Most common endocrinopathy in McCune-Albright syndrome, particularly affecting girls
- Occurs due to autonomous ovarian function with follicular cyst formation
- Median age of onset: 4.8 years 3
- Results from mosaic GNAS mutations causing autonomous activation of gonadal tissue
- May require resection of ovarian cysts in severe cases 4
Growth Hormone Excess
- Occurs in 5-21% of patients with McCune-Albright syndrome 1, 5
- Typically presents with early-onset GH excess starting around age 3 years onwards 6
- Almost half (46%) are children at time of diagnosis 6
- Clinical features include:
- Accelerated growth velocity (>2 SDS)
- Acral enlargement
- Coarsened facial features and prognathism
- Café-au-lait pigmentation
- Fibrous dysplasia (especially craniofacial bones)
- Critical management consideration: Radiotherapy should be avoided due to higher risk of sarcomatous transformation in dysplastic bone (6 of 112 patients developed skull base sarcoma, with 3 having prior pituitary irradiation) 6
- Treatment relies on medical therapy (somatostatin analogues, pegvisomant) or surgery rather than radiotherapy 5
Hyperthyroidism
- Occurs in approximately 8 patients among reported cohorts 3
- Presents as toxic nodular thyroid disease with primary hyperthyroidism 4
- Results from autonomous thyroid tissue activation
- Requires standard hyperthyroidism management
Hypercortisolism
- Less common manifestation
- Results from autonomous adrenal cortical function
- Presents with typical Cushing syndrome features
- Reported in 2 patients with cortisol excess in recent cohorts 3
Hypophosphatemic Rickets/Phosphate Wasting
- Occurs in approximately 6 patients in documented series 3
- Results from renal phosphate wasting
- Contributes to skeletal complications beyond fibrous dysplasia
- Requires phosphate supplementation and active vitamin D therapy
Hyperprolactinemia
- Rare manifestation in McCune-Albright syndrome
- Can cause hypogonadotropic hypogonadism and premature menopause 4
- May coexist with GH excess (34-36% of patients with gigantism have prolactin co-secretion) 6
Hyperparathyroidism
- Uncommon but documented manifestation
- Presents as primary hyperparathyroidism 4, 7
- May progress to tertiary hyperparathyroidism in chronic cases
- Associated with hypercalcemia, kidney stones, and osteoporosis
Screening Recommendations
All children and young people with McCune-Albright syndrome require biochemical screening for pituitary hormone excess (strong recommendation) 6:
- IGF-1 levels (age-adjusted, sex-adjusted, Tanner stage-matched)
- GH levels with oral glucose tolerance test if GH excess suspected
- Prolactin levels
- Thyroid function tests (TSH, free T4)
- Morning cortisol and ACTH
- Calcium, phosphate, PTH, and 25-OH vitamin D
- Gonadotropins and sex steroids (assess for precocious puberty)
Key Clinical Pitfalls
GH excess may be masked by precocious puberty or craniofacial fibrous dysplasia, necessitating active screening 5
Avoid radiotherapy in patients with craniofacial fibrous dysplasia due to increased sarcomatous transformation risk 6
Multiple endocrinopathies can coexist in the same patient - one case report documented at least four primary endocrinopathies simultaneously 4
Severe manifestations can present in neonates with complex symptoms that may not immediately suggest McCune-Albright syndrome 8
Adult-onset endocrinopathies can occur, including acromegaly presenting in the fourth decade, emphasizing need for lifelong surveillance 9, 7
Pathophysiology
All endocrine manifestations result from mosaic somatic activating mutations in the GNAS gene (encoding Gsα protein), causing constitutive activation of cyclic AMP signaling in affected tissues 1, 2. The variable distribution of mutated cells explains the heterogeneous clinical presentation across different endocrine organs.