A 28-year-old man presents with palpitations, tremor, heat intolerance, and weight loss for 2 weeks. TSH is suppressed at 0.05 mIU/L (normal 0.4–4.0), and free T4 is elevated at 28 pmol/L (normal 10–20). To differentiate between Graves' disease, thyroiditis, and thyroid hormone ingestion, which investigation is most appropriate?

## Differential Diagnosis of Thyrotoxicosis **Key Point:** In thyrotoxicosis with suppressed TSH and elevated free T4, radioactive iodine uptake (RAIU) scan is the gold standard investigation to differentiate between thyroid-autonomous conditions (Graves', toxic nodule) and non-autonomous causes (thyroiditis, exogenous hormone ingestion). ### Mechanism of RAIU in Differential Diagnosis The thyroid iodine uptake reflects thyroid-stimulating immunoglobulin (TSI) activity and thyroid avidity for iodine: | Condition | RAIU Uptake | TSH | Free T4 | Mechanism | |---|---|---|---|---| | **Graves' disease** | **High** (>30%) | Low | High | TSI-driven autonomous production | | **Toxic nodule** | **High** (focal) | Low | High | Autonomous hormone production | | **Thyroiditis** | **Low** (<5%) | Low | High | Preformed hormone release, no new synthesis | | **Exogenous hormone** | **Low** (<5%) | Low | High | Suppressed endogenous uptake | **High-Yield:** RAIU is the ONLY investigation that directly measures thyroid iodine avidity and distinguishes autonomous from non-autonomous thyrotoxicosis. ### Why RAIU is Superior 1. **Functional assessment:** Directly measures thyroid's ability to trap and utilize iodine 2. **Pathophysiologic distinction:** High uptake = thyroid is producing hormone; low uptake = thyroid is not producing (thyroiditis) or exogenous hormone is suppressing uptake 3. **Guides management:** Graves' disease and toxic nodules require antithyroid drugs or ablation; thyroiditis is self-limited ### Role of Other Investigations | Investigation | Finding | Limitation | |---|---|---| | **TPO/Tg antibodies** | Positive in autoimmune thyroiditis, Graves' disease | Does NOT differentiate Graves' from thyroiditis; both can have positive antibodies | | **Serum thyroglobulin** | Elevated in thyroiditis (hormone release); suppressed in Graves' (TSI suppresses TSH → low Tg synthesis) | Overlap; not reliable for differentiation | | **Thyroid ultrasound** | Shows vascularity in Graves' (increased blood flow); hypoechoic in thyroiditis | Structural finding; does not assess functional iodine uptake | **Clinical Pearl:** In thyroiditis, thyroglobulin is RELEASED from damaged follicles (high serum Tg initially), but the thyroid is NOT synthesizing new hormone. In Graves' disease, TSI drives continuous synthesis and release. RAIU directly distinguishes these mechanisms. **Mnemonic:** **RAIU = Rapid Assessment of Iodine Uptake** — high uptake = autonomous thyroid (Graves', toxic nodule); low uptake = non-autonomous (thyroiditis, exogenous hormone). **Tip:** NEET PG frequently tests the differential of thyrotoxicosis. Remember: **RAIU is the investigation of choice to differentiate autonomous from non-autonomous thyrotoxicosis.**