A 32-year-old woman presents with recurrent, unexpected panic attacks characterized by palpitations, dyspnea, chest pain, and fear of dying. She has been avoiding public places and crowded settings due to fear of having another attack in situations where escape might be difficult. Physical examination and routine blood work are normal. Which investigation is most appropriate to exclude organic causes before confirming panic disorder?

Explanation

## Investigation of Choice in Suspected Panic Disorder ### Why Thyroid Function Tests and Serum Calcium? **Key Point:** Panic disorder is a diagnosis of exclusion. Organic causes mimicking panic attacks—particularly thyroid dysfunction (hyperthyroidism) and hypocalcemia—must be ruled out before confirming the psychiatric diagnosis. ### Differential Diagnosis: Organic Mimics of Panic Disorder | Condition | Key Features | Investigation | |-----------|--------------|----------------| | Hyperthyroidism | Palpitations, tremor, anxiety, heat intolerance | TSH (↓), free T4 (↑) | | Hypocalcemia | Paresthesias, tetany, anxiety, palpitations | Serum calcium, ionized Ca²⁺ | | Cardiac arrhythmia | Palpitations, chest pain, syncope | ECG, Holter (if paroxysmal) | | Panic disorder (primary) | Unexpected attacks, anticipatory anxiety, no organic cause | Clinical diagnosis after exclusion | **High-Yield:** TSH is the single most sensitive screening test for thyroid disorders. Hyperthyroidism is the most common endocrine mimic of panic disorder. ### Clinical Pearl In this case, routine blood work is reported as normal, but the question asks for the **most appropriate investigation**—implying a systematic approach to exclusion. TSH and serum calcium are first-line screening tests in the workup of suspected panic disorder, as per DSM-5 and ICD-10 guidelines. They are non-invasive, cost-effective, and detect the most common organic mimics. ### Diagnostic Approach ```mermaid flowchart TD A[Suspected Panic Disorder]:::outcome --> B{Organic causes excluded?}:::decision B -->|No| C[TSH, free T4, serum calcium]:::action B -->|Yes| D[Assess for cardiac arrhythmia]:::decision D -->|Paroxysmal symptoms| E[24-hour Holter monitor]:::action D -->|No arrhythmia risk| F[Confirm panic disorder clinically]:::outcome C --> G{Abnormal?}:::decision G -->|Yes| H[Treat underlying condition]:::action G -->|No| F ``` **Mnemonic:** **CATCH** the organic cause — **C**alcium, **A**drenergic (thyroid), **T**emperature sensitivity (thyroid), **C**ardiac (ECG if indicated), **H**ypoglycemia (glucose).