Schmidt syndrome

**Definition / Classification** - Schmidt syndrome = Autoimmune Polyglandular Syndrome type 2 (APS-2). Classic definition: Addison disease + autoimmune thyroid disease (Hashimoto thyroiditis or Graves) and/or type 1 DM. Addison disease is required. - Carpenter syndrome is the older name for the combination of Addison disease + T1DM. Schmidt syndrome by strict definition is Addison + autoimmune thyroid disease. - Other associations: primary hypogonadism (premature ovarian failure), vitiligo, celiac disease, pernicious anemia/autoimmune gastritis, alopecia areata, myasthenia gravis, and hypophysitis.

**Pathophysiology** - Polygenic autoimmune disorder with T-cell-mediated and autoantibody-mediated destruction of endocrine organs. - Strong HLA association: HLA-DR3 and HLA-DR4 (also CTLA-4 polymorphisms). Not caused by a single-gene defect. - Adrenal autoantibodies: anti-21-hydroxylase. Thyroid: anti-TPO, anti-thyroglobulin, and TSH receptor antibodies (Graves). T1DM: anti-GAD65, anti-insulin, and anti-islet cell antibodies.

**Clinical features** - Typical onset in adulthood (ages 20-40), female predominance. Family clustering with autosomal dominant-like inheritance and incomplete penetrance. - Primary adrenal insufficiency: fatigue, weight loss, hypotension, hyperpigmentation (↑ACTH/POMC), hyponatremia, hyperkalemia, non-anion gap metabolic acidosis, hypoglycemia, and salt craving. - Thyroid disease: hypothyroidism (Hashimoto) or hyperthyroidism (Graves). - T1DM: polyuria, polydipsia, weight loss, and DKA risk.

**Diagnosis** - Low AM cortisol with high ACTH, and failure to respond to the cosyntropin (ACTH) stimulation test. - Elevated renin with low aldosterone. - Screen for other components: TSH, anti-TPO, glucose/HbA1c, and anti-21-hydroxylase antibodies. - Monitor family members and patients with one component for the others.

**Management (high-yield pearls)** - Replace glucocorticoids (hydrocortisone) and mineralocorticoids (fludrocortisone). - Start glucocorticoid BEFORE levothyroxine. Thyroid hormone increases cortisol clearance and metabolic demand and can precipitate adrenal crisis in untreated Addison disease. - Insulin for T1DM. Insulin requirements may fall and hypoglycemia risk rises with adrenal insufficiency. - Give stress-dose steroids during illness, surgery, or trauma.

**APS comparison (frequently tested)** - APS-1 (APECED): AIRE gene mutation (chromosome 21, autosomal recessive), presenting in childhood. Triad: chronic mucocutaneous candidiasis, hypoparathyroidism, and adrenal insufficiency. Other findings: enamel hypoplasia, alopecia, and hypogonadism. - APS-2 (Schmidt): polygenic, HLA-DR3/DR4, adult onset. Addison + thyroid autoimmunity ± T1DM. No candidiasis or hypoparathyroidism. - APS-3: autoimmune thyroid disease + another autoimmune disease (often T1DM), but NO Addison disease. - IPEX (APS-4-like): FOXP3 mutation, X-linked, with immune dysregulation, polyendocrinopathy (neonatal T1DM), enteropathy.

**Don't confuse with** - Schmidt syndrome is also an eponym for a vagal/laryngeal paralysis syndrome (unilateral paralysis of the soft palate, vocal cord, trapezius, and sternocleidomastoid from CN X and XI lesions). On endocrine-focused boards, "Schmidt syndrome" almost always means APS-2.