A 38-year-old woman presents with fatigue, weight gain, cold intolerance, and dry skin for 6 months. On examination, she has a slow pulse (52/min) and delayed relaxation of deep tendon reflexes. Which investigation is most appropriate to confirm the diagnosis of hypothyroidism?

Explanation

## Confirmatory Investigation for Hypothyroidism **Key Point:** Serum TSH is the most sensitive and specific screening and confirmatory test for primary hypothyroidism. It is elevated in hypothyroidism and is the gold standard first-line investigation. ### Why TSH is the Investigation of Choice **High-Yield:** TSH has an inverse relationship with thyroid hormone levels. In primary hypothyroidism: - TSH is **markedly elevated** (>4.0–5.0 mIU/L, often >10 mIU/L) - Free T4 is **low** (<0.8 ng/dL) - This combination is diagnostic of primary hypothyroidism **Clinical Pearl:** TSH is more sensitive than free T4 for detecting early or subclinical hypothyroidism, where free T4 may still be in the low-normal range but TSH is already elevated. ### Investigation Algorithm for Suspected Hypothyroidism ```mermaid flowchart TD A[Clinical suspicion of hypothyroidism]:::outcome --> B[Measure serum TSH]:::action B --> C{TSH elevated?}:::decision C -->|Yes| D[Measure free T4]:::action D --> E{Free T4 low?}:::decision E -->|Yes| F[Primary hypothyroidism confirmed]:::outcome E -->|No| G[Subclinical hypothyroidism]:::outcome C -->|No| H[Rule out hypothyroidism]:::outcome ``` ### Role of Other Investigations | Investigation | Role in Hypothyroidism | When Used | |---|---|---| | **Free T3** | Not useful for diagnosis; may be normal or low | Not routinely done; only in special cases | | **TPO antibodies** | Identifies autoimmune etiology (Hashimoto's) | Confirms autoimmune thyroiditis, not diagnostic of hypothyroidism | | **Thyroid ultrasound** | Shows structural changes (echogenicity, size) | When thyroid nodule or goiter present; not for diagnosis confirmation | **Mnemonic:** **TSH-FT4 Duo** — Always measure TSH first; if elevated, confirm with free T4 to diagnose primary hypothyroidism. **Warning:** Do not rely on clinical symptoms alone — they overlap with many conditions (depression, anemia, obesity). TSH + free T4 are mandatory for diagnosis.