A 28-year-old woman delivers a healthy infant 6 weeks ago and now presents with progressive fatigue, weight gain, and difficulty concentrating. She denies neck pain or fever. On examination, the thyroid is diffusely enlarged, firm, and non-tender. TSH is 9.5 mIU/L, free T4 is 1.2 ng/dL (low-normal), TPO antibodies are 450 IU/mL (normal <35), and thyroglobulin antibodies are 280 IU/mL (normal <20). Thyroid ultrasound shows heterogeneous echotexture with preserved vascularity on Doppler. What is the most likely diagnosis?

Explanation

## Clinical Diagnosis: Postpartum Thyroiditis ### Definition and Epidemiology **Key Point:** Postpartum thyroiditis is an autoimmune thyroid inflammation occurring within 12 months of delivery, affecting 5–10% of postpartum women. It is the most common cause of thyroid dysfunction in the postpartum period. ### Pathophysiology Postpartum thyroiditis results from a rebound in cell-mediated immunity after pregnancy-induced immune suppression: 1. **Pregnancy:** Th2-dominant (humoral) immunity predominates; Th1 (cellular) immunity suppressed 2. **Postpartum:** Rapid shift back to Th1 dominance → autoimmune attack on thyroid 3. **Result:** Lymphocytic infiltration, follicular disruption, and release of preformed hormone ### Clinical Phases | Phase | Timing | TSH | Free T4 | Symptoms | Duration | | --- | --- | --- | --- | --- | --- | | **Thyrotoxic** | Weeks 2–4 | Low | High | Palpitations, anxiety, heat intolerance | 2–8 weeks | | **Hypothyroid** | Weeks 4–12 | High | Low | Fatigue, weight gain, depression | 4–6 months | | **Recovery** | Months 3–12 | Normal | Normal | Resolution | Variable | **High-Yield:** Not all women progress through all phases; some have only thyrotoxicosis or only hypothyroidism. ### Why This Patient Has Postpartum Thyroiditis **Timing:** 6 weeks postpartum (within the typical 2–12 month window) **Clinical presentation:** Hypothyroid phase with: - Fatigue, weight gain, difficulty concentrating - Elevated TSH (9.5) with low-normal T4 - **Positive TPO and thyroglobulin antibodies** → autoimmune etiology - **Non-tender, diffusely enlarged thyroid** → lymphocytic infiltration, not acute inflammation **Ultrasound findings:** Heterogeneous echotexture (lymphocytic infiltration) with preserved vascularity (not the suppressed uptake of subacute thyroiditis) ### Distinguishing Features from Other Thyroiditis | Feature | Postpartum | Hashimoto | Subacute Granulomatous | Graves | | --- | --- | --- | --- | --- | | **Onset** | 2–12 months postpartum | Insidious, any age | Acute, often post-viral | Acute/subacute | | **Pain** | None | None | Severe, tender | None | | **Antibodies** | TPO/Tg positive | TPO/Tg positive | Negative | TSI positive | | **ESR** | Normal/mildly elevated | Normal | Markedly elevated (>50) | Normal | | **RAIU** | Low (suppressed) | Low | Low (suppressed) | High (increased) | | **Thyroid vascularity** | Normal | Normal | Reduced | Increased | | **Course** | Triphasic, often self-limited | Progressive | Self-limited in weeks | Progressive | **Clinical Pearl:** Postpartum thyroiditis is often confused with postpartum depression due to similar symptoms (fatigue, mood changes, poor concentration). However, the presence of thyroid dysfunction on labs distinguishes it. Screening TSH is recommended in all postpartum women with mood or energy disturbances. **Mnemonic: POSTPARTUM THYROIDITIS = PPTD** - **P**ostpartum timing (2–12 months) - **P**ositive antibodies (TPO, Tg) - **T**riphasic course (thyrotoxic → hypothyroid → recovery) - **D**iffuse, non-tender enlargement ### Management - **Hypothyroid phase:** Levothyroxine replacement (start 25–50 mcg/day, titrate to TSH 0.5–2.0) - **Thyrotoxic phase (if symptomatic):** Beta-blockers; avoid antithyroid drugs (not indicated) - **Monitoring:** TSH every 6–8 weeks; most recover by 12 months - **Breastfeeding:** Safe to continue; levothyroxine minimally excreted in breast milk **Warning:** Do NOT treat the thyrotoxic phase with PTU or methimazole — this is NOT Graves disease. The thyrotoxicosis is due to hormone release, not increased synthesis. ### Long-term Risk - 20–30% develop permanent hypothyroidism and require lifelong levothyroxine - Increased risk of recurrence in subsequent pregnancies (25–50%) - Some progress to Hashimoto thyroiditis [cite:Robbins 10e Ch 24; Harrison 21e Ch 405]