A case report to be presented at the American Association of Clinical Endocrinology (AACE) Annual Meeting 2026 describes a diagnosis of systemic mastocytosis (SM) made after a routine referral for hypothyroidism (an underactive thyroid).
SM can affect several body systems, causing symptoms that vary widely, overlap with other disorders and contribute to delayed diagnosis. “This case highlights the importance of considering a broad differential diagnosis even with common conditions, such as hypothyroidism,” the author wrote.
In the abstract, researchers described a 37-year-old woman who presented for routine hypothyroidism management with fatigue, constipation, weight gain and brain fog. However, her symptoms extended beyond typical thyroid disease and included diffuse muscle pain, tachycardia (elevated heart rate), episodic flushing, lightheadedness and an itchy rash on her face, chest and back.
The broader symptom pattern led clinicians to order additional testing instead of attributing all symptoms to thyroid disease.
The diagnostic workup found an elevated serum tryptase level of 46.8 μg/L, a marker of mast cell activity used to evaluate mast cell disorders. Other blood tests did not indicate thyroid, adrenal, autoimmune or connective tissue abnormalities. Flow cytometry, a lab test used to analyze cells, revealed atypical expression of CD2 and CD25, cell-surface proteins that can help identify mast cells involved in SM.
Skin biopsies revealed small blood vessel changes and mild inflammation. A bone marrow biopsy identified abnormal mast cells, which accounted for 10% to 20% of cells and often had an unusual shape. Together, these findings supported the SM diagnosis.
Read more about SM testing and diagnosis
This case reflects a common challenge in rare diseases: symptoms may involve different body systems and appear unrelated at first. In SM, mast cells can release substances that affect the skin, heart rate, blood pressure, gastrointestinal system and other parts of the body, which can make the condition difficult to recognize.
Earlier diagnosis may help patients move from unexplained symptoms toward appropriate monitoring and symptom management. It can also help clinicians assess the risk of severe allergic reactions, evaluate for organ involvement and determine whether referral to specialists familiar with mast cell disorders is needed.
Sign up here to get the latest news, perspectives, and information about SM sent directly to your inbox. Registration is free and only takes a minute.
