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Familial glucocorticoid deficiency
Familial glucocorticoid deficiency (FGD) is a group of primary adrenal insufficiencies characterized clinically by neonatal hyperpigmentation, hypoglycemia, failure to thrive, and recurrent infections, and biochemically by glucocorticoid deficiency without mineralocorticoid deficiency.
The prevalence is unknown. In Ireland there is a prevalence of around 1/200,000, but this is likely to be skewed by a high prevalence in the Irish Traveler sub-population.
FGD usually presents in infancy or early childhood with hyperpigmentation of the skin and gums (present at birth or that develops over time), hypoglycemic seizures and failure to thrive. Recurrent infections are also a common finding (and may be the presenting sign in older children). Weakness, fatigue, weight loss, anorexia, vomiting, flank or abdominal pain, constipation and diarrhea are additional symptoms seen in some patients due to hypocortisolemia. Hypoglycemic crises resulting in convulsions can lead to coma or death if untreated and recurrent hypolglycemia may lead to neurological sequelae (i.e. learning disabilities, intellectual deficit, and sometimes severe, neuronal damage leading to major sensory and motor defects such as quadriplegia). Tall stature has been reported in some patients with FGD, typically those with MC2R gene defects. MRAP defects have been associated with a more severe disease and an earlier age of onset while a milder phenotype is seen in those with defects in the MCM4 gene (Irish Traveler FGD).
FGD is due, in most cases, to defects in the adrenocorticotropin (ACTH) receptor, or its signaling pathway, resulting in a failure of the cells of zona fasciculata in the adrenal cortex to respond appropriately to adrenocorticotrophic hormone (ACTH), leading to a glucocorticoid deficiency. These defects are most commonly caused by mutations in MC2R (18p11.2), accounting for 25% of cases, and MRAP (21q22.1), accounting for 20% of cases. Other mutations reported in patients with FGD include MCM4 (8q12-q13), probably uniquely in the Irish Traveler population; NNT (5p12), accounting for around 15% of cases; and TXNRD2 (22q11.21). Certain partially inactivating mutations of STAR (8p11.2) or CYP11A1 (15q23-q24) can cause a phenotype that masquerades as FGD.
Diagnosis is based on clinical and laboratory findings. Patients have high plasma ACTH and low serum morning cortisol levels that do not respond to exogenous ACTH stimulation. Mineralocorticoid function is normal. Molecular genetic testing revealing a mutation in one of the disease causing genes confirms diagnosis of FGD.
The main differential diagnosis of FGD is Addison's disease (usually of autoimmune origin), in which case a mineralocorticoid deficiency is present. Other differential diagnoses include triple A syndrome, congenital adrenal hyperplasia and other acquired causes of primary adrenal insufficiency (see these terms).
Prenatal diagnosis is possible in families with a known disease causing mutation but is rarely performed.
FDG is inherited in an autosomal recessive manner. Genetic counseling is possible.
Management and treatment
Treatment consists of a replacement therapy with oral hydrocortisone. A dosage of 10-12 mg/m2/day (usually divided into three doses) normalizes cortisol and reduces, but rarely normalizes, ACTH. Dose modification is necessary during stresses such as surgery or intercurrent illness, and patients should have injectable hydrocortisone available for emergencies and carry a medical alert type bracelet or card. Prompt and adequate treatment of a hypoglycemic crisis is essential. Treatment is life-long.
The prognosis is good for patients who are diagnosed and treated early. Only when left untreated is FGD a disease with high morbidity (neurological sequelae) and mortality.