In the management of subacute postpartum thyroiditis, which clinical feature distinguishes it from Graves' hyperthyroidism postpartum, and what is the key difference in radioactive iodine uptake (RAIU) test results?

• A Both have elevated T3/T4 and elevated RAIU; distinguished by TRAb antibodies in Graves
• B Postpartum thyroiditis: low/suppressed RAIU (destructive thyroiditis); Graves' postpartum: elevated RAIU with elevated TRAb ✓
• C Postpartum thyroiditis: elevated TSH with RAIU suppressed; Graves: normal TSH with elevated T4
• D Both have low RAIU; distinguished by TPO antibodies in thyroiditis vs TRAb in Graves

Explanation

Postpartum thyroiditis is a lymphocytic (autoimmune) destructive thyroiditis occurring within 1 year of delivery, causing a transient hyperthyroid phase (weeks 1–4 postpartum) due to preformed thyroid hormone release from thyroid follicle destruction — not de novo synthesis. Because thyroid tissue is being destroyed rather than overstimulated, RAIU is low or suppressed (< 1–2% at 24 hours). Graves' disease (postpartum flare) is driven by stimulatory TSH-receptor antibodies (TRAb/TSHR-Ab) causing active hormone synthesis — RAIU is high (> 30–50%). TPO antibodies are positive in both but TSH-receptor stimulating antibodies (TRAb) are diagnostic of Graves'. RAIU is the key differentiating test.

Reference: Williams Obstetrics, 26th ed.

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