## Diagnostic Approach to Hypothyroidism **Key Point:** TSH is the single best screening and confirmatory test for primary hypothyroidism due to its exquisite sensitivity and inverse relationship with thyroid hormone levels. ### Why TSH is the Gold Standard TSH (thyroid-stimulating hormone) is produced by the anterior pituitary and follows a negative feedback loop with thyroid hormones. In primary hypothyroidism: - TSH rises significantly (typically >10 mIU/L) as the pituitary attempts to stimulate the failing thyroid - This rise occurs BEFORE free T4 falls below the normal range (subclinical hypothyroidism phase) - TSH is therefore more sensitive than FT4 for early detection ### Diagnostic Algorithm ```mermaid flowchart TD A[Clinical suspicion of hypothyroidism]:::outcome --> B[Measure TSH]:::action B --> C{TSH elevated?}:::decision C -->|Yes| D[Measure Free T4]:::action C -->|No| E[Hypothyroidism unlikely]:::outcome D --> F{FT4 low?}:::decision F -->|Yes| G[Overt hypothyroidism]:::outcome F -->|No| H[Subclinical hypothyroidism]:::outcome ``` ### Interpretation of Results | TSH Level | Free T4 | Diagnosis | |-----------|---------|----------| | Elevated (>10) | Low | Overt primary hypothyroidism | | Elevated (4.5–10) | Normal | Subclinical hypothyroidism | | Normal or low | Low | Central (secondary) hypothyroidism | | Normal | Normal | Euthyroid | **High-Yield:** TSH alone is sufficient to diagnose and monitor primary hypothyroidism in most clinical scenarios. FT4 is measured only if TSH is abnormal to confirm the diagnosis and assess severity. **Clinical Pearl:** The delayed ankle jerk reflex (Achilles reflex) described in this patient is a classic sign of hypothyroidism due to slowed muscle relaxation — a bedside clue that should prompt immediate TSH measurement.
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