## Preparation for Radioactive Iodine Therapy in PTC ### Mechanism of RAI Uptake Enhancement **Key Point:** Thyroid-stimulating hormone (TSH) is the primary driver of iodine uptake by thyroid follicular cells. To maximize RAI concentration in residual thyroid tissue and metastases, TSH must be elevated before RAI administration. ### Why Liothyronine (T3) Withdrawal? 1. **Liothyronine has a short half-life (~1.5 days)** compared to levothyroxine (T4, ~7 days). 2. **Rapid TSH rebound** occurs within 3–5 days of T3 withdrawal, allowing high TSH levels to drive iodine uptake. 3. **Minimal hypothyroid symptoms** during the brief withdrawal period (compared to T4 withdrawal, which causes prolonged hypothyroidism). 4. **Standard protocol:** T3 is given for 2 weeks post-operatively, then stopped to allow TSH to rise before RAI administration. ### Comparison of TSH Suppression Strategies | Strategy | TSH Elevation Timeline | Symptom Burden | Clinical Use | | --- | --- | --- | --- | | T3 withdrawal | 3–5 days | Minimal | **Preferred for RAI prep** | | T4 withdrawal | 4–6 weeks | Severe hypothyroidism | Rarely used now | | Recombinant TSH (rhTSH) | Immediate (by injection) | None | Alternative in select cases | | T4 alone (suppressive) | Suppressed (TSH <0.1) | Iatrogenic hyperthyroidism | Long-term surveillance, not RAI prep | **High-Yield:** The **2-week T3 protocol** is the gold standard in most centers because it balances rapid TSH elevation with patient tolerance. ### Clinical Pearl After RAI therapy, patients transition to **levothyroxine suppressive therapy** (target TSH 0.1–0.5 mIU/L) for 5–10 years to inhibit any residual thyroid cancer cell growth. This is NOT the same as the pre-RAI preparation phase. [cite:Harrison 21e Ch 397]
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