## Diagnostic Approach to Hyperthyroidism **Key Point:** TSH is the most sensitive screening test for thyroid dysfunction and is the single best initial investigation for confirming hyperthyroidism. ### Why TSH First? In hyperthyroidism, excess thyroid hormones suppress the pituitary via negative feedback, causing **suppressed TSH** (typically <0.1 mIU/L). This is the earliest and most sensitive indicator of thyroid hormone excess. ### Diagnostic Algorithm ```mermaid flowchart TD A[Clinical suspicion of hyperthyroidism]:::outcome --> B[Measure TSH]:::action B --> C{TSH suppressed?}:::decision C -->|Yes| D[Measure Free T4 and Free T3]:::action C -->|No| E[TSH normal/elevated<br/>Unlikely hyperthyroidism]:::outcome D --> F{Elevated Free T4<br/>and/or Free T3?}:::decision F -->|Yes| G[Hyperthyroidism confirmed]:::outcome F -->|No| H[Subclinical hyperthyroidism<br/>or TSH-secreting pituitary]:::outcome ``` ### TSH vs Free T4/T3 | Investigation | Sensitivity | Specificity | Use | |---|---|---|---| | **TSH** | Highest | High | Screening test of choice | | **Free T4** | Lower | High | Confirmatory after abnormal TSH | | **Free T3** | Lower | Moderate | Useful in T3 toxicosis; not first-line | **High-Yield:** TSH is suppressed in *all* causes of true hyperthyroidism (Graves, toxic nodule, thyroiditis, exogenous hormone). It is the **gatekeeper** investigation. ### Clinical Pearl In this patient with clinical signs of hyperthyroidism (palpitations, tremor, weight loss, goitre, tachycardia), a suppressed TSH (<0.1 mIU/L) immediately confirms the biochemical diagnosis. Free T4/T3 measurement follows to quantify severity and guide treatment. [cite:Harrison 21e Ch 397]
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