Radioiodine, Targeted Therapy, and Special Contexts
Thyroid cancer pharmacology, amiodarone thyroid disease, and pregnancy
Radioactive Iodine and Thyroid-Stimulating Hormone Suppression in Thyroid Cancer
Pre-RAI Prep
TSH Stimulation Methods
  • rhTSH (thyrotropin alfa): 0.9 mg IM ×2 days; RAI on day 3
  • Maintains euthyroidism; preferred low-to-intermediate risk
  • Withdrawal: 4 weeks off levothyroxine; required high-risk + dosimetry
  • Low-iodine diet required regardless of method
TSH Targets
Risk-Stratified Suppression
  • High risk (metastases): TSH <0.1 mIU/L
  • Intermediate risk: TSH 0.1–0.5 mIU/L
  • Low risk after excellent response: TSH 0.5–2.0 mIU/L
  • De-escalate at earliest opportunity
Suppression Harms
Long-Term Risks
  • Bone: reduced mineral density; fracture risk (postmenopausal)
  • Heart: atrial fibrillation risk 2–3× in patients over 60
  • Monitor bone density; screen for atrial fibrillation annually
Amiodarone-Induced Thyrotoxicosis: Type 1 vs. Type 2
Feature Type 1 (Iodine-Induced) Type 2 (Destructive)
Mechanism Iodine excess drives autonomous synthesis Direct cytotoxicity releases preformed hormone
Thyroid anatomy Pre-existing goiter or nodular disease Normal or mildly enlarged gland
Color Doppler Increased vascularity Absent or markedly reduced vascularity
Treatment High-dose methimazole ± perchlorate Glucocorticoids (prednisone 40 mg/day, taper)
Thyroid Disease in Pregnancy
Graves’ Disease
Thionamide Goals
  • Target: free T4 in upper third of reference range
  • Use lowest effective dose — fetus exposed to thionamide
  • Propylthiouracil in 1st trimester; switch to methimazole at 16 weeks
  • Block-and-replace contraindicated — excess fetal thionamide exposure
  • Check thyroid function every 4 weeks; adjust dose to target
  • TRAb at 28–32 weeks predicts neonatal Graves’ risk
Neonatal Graves’
The 3–7 Day Delay
  • Maternal TRAb cross placenta; drive neonatal thyroid
  • Maternal antithyroid drug clears in 3–7 days postpartum
  • Thyrotoxicosis emerges as drug clears, TRAb persist
  • Normal newborn screen does not exclude delayed-onset disease
  • Treatment: methimazole 0.2–0.5 mg/kg/day + propranolol
  • Self-limited: TRAb clear over 3–6 months
Key Rule: Amiodarone Expected Pattern Is Not Disease

In the first 3 months of amiodarone therapy: elevated free T4, low T3, high reverse T3, and transiently elevated TSH is a pharmacological drug effect — not thyroid disease. Do not treat. True amiodarone-induced thyrotoxicosis is diagnosed when free T4 remains elevated beyond this window with suppressed TSH and clinical features of thyrotoxicosis.