A 32-year-old woman presents with sudden onset thyroid pain, fever, and dysphagia. On examination, the thyroid is tender and enlarged. TSH is suppressed at 0.1 mIU/L, free T4 is elevated at 18 pg/dL, and thyroid peroxidase (TPO) antibodies are negative. Which investigation is most specific for confirming the diagnosis of subacute thyroiditis?

## Diagnosis of Subacute Thyroiditis **Key Point:** Subacute (de Quervain's) thyroiditis is a self-limited inflammatory condition characterized by a triphasic course: thyrotoxicosis → euthyroidism → hypothyroidism. The clinical presentation and biochemistry (suppressed TSH, elevated free T4, negative TPO antibodies) are highly suggestive, but the **thyroid radionuclide uptake scan** is the most specific confirmatory investigation. ## Why Radionuclide Uptake is Diagnostic In subacute thyroiditis, the inflammatory destruction of thyroid follicles releases preformed thyroid hormones into the circulation, causing thyrotoxicosis. However, the **thyroid iodine uptake is markedly reduced or absent** because: 1. TSH is suppressed due to circulating thyroid hormones 2. The inflamed thyroid tissue does not actively trap iodine 3. This **low uptake in the setting of thyrotoxicosis** is pathognomonic for subacute thyroiditis In contrast, Graves' disease (the main differential) shows **high uptake** despite thyrotoxicosis. ## Differential Uptake Pattern | Condition | TSH | Free T4 | Uptake | TPO Ab | |-----------|-----|---------|--------|--------| | Subacute thyroiditis | ↓ | ↑ | **Very low** | Negative | | Graves' disease | ↓ | ↑ | **High** | Negative | | Hashimoto's (acute phase) | ↓ | ↑ | Low | **Positive** | | Thyroid hormone ingestion | ↓ | ↑ | **Very low** | Negative | **Clinical Pearl:** The low radionuclide uptake in the presence of thyrotoxicosis is the **gold standard** for distinguishing subacute thyroiditis from Graves' disease and thyroid hormone factitia. This is the most specific test for confirming the diagnosis. **High-Yield:** Remember the **"painful thyroid + low uptake + thyrotoxicosis = subacute thyroiditis"** triad. Radionuclide imaging is essential when the clinical picture is unclear or to confirm the diagnosis before starting corticosteroids.