A 28-year-old Indian woman, 6 weeks postpartum, presents with palpitations, fatigue, and insomnia. She denies heat intolerance or weight loss. On examination, heart rate is 98 bpm, thyroid is not enlarged, and there is no exophthalmos. TSH is 0.05 mIU/L (normal 0.4–4.0), free T4 is 18 pg/dL (normal 7.5–20), and free T3 is 4.8 pg/mL (normal 2.3–4.2). Thyroid peroxidase (TPO) antibodies are positive at 250 IU/mL (normal <35). Thyroid uptake scan shows suppressed uptake. What is the most likely diagnosis?

Explanation

## Clinical Diagnosis: Postpartum Thyroiditis (Thyrotoxic Phase) ### Key Diagnostic Features **Key Point:** Postpartum thyroiditis is an autoimmune inflammation of the thyroid occurring in 5–10% of postpartum women. It typically presents in two phases: thyrotoxic (weeks 1–4) and hypothyroid (weeks 4–12), though some patients experience only one phase. ### Clinical Presentation This patient exhibits the hallmark features of postpartum thyroiditis: 1. **Timing**: 6 weeks postpartum (classic window: 1–6 months) 2. **Symptoms**: Palpitations, fatigue, insomnia — consistent with mild thyrotoxicosis 3. **Absence of heat intolerance and weight loss**: Suggests thyroid hormone *release* (not synthesis), which causes milder systemic symptoms than Graves' disease 4. **No goiter, no exophthalmos**: Rules out Graves' disease 5. **Positive TPO antibodies**: Indicates autoimmune thyroiditis ### Diagnostic Confirmation | Feature | Postpartum Thyroiditis | Graves' Disease | |---------|------------------------|------------------| | TSH | Suppressed | Suppressed | | Free T4 | Normal to mildly ↑ | Markedly ↑ | | Free T3 | Mildly ↑ | Markedly ↑ | | **Thyroid uptake** | **Suppressed** | **Increased** | | TPO antibodies | Positive | Negative | | Goiter | Absent or mild | Diffuse | | Exophthalmos | Absent | May be present | | Symptom severity | Mild | Moderate to severe | **High-Yield:** The **suppressed thyroid uptake** is the key differentiator. In postpartum thyroiditis, the gland is inflamed and leaking stored hormone — it is NOT synthesizing new hormone, so uptake is low. In Graves' disease, the gland is actively synthesizing hormone, so uptake is high. ### Mechanism Postpartum thyroiditis is thought to result from: 1. **Rebound autoimmunity** after the immunosuppression of pregnancy 2. **Molecular mimicry** triggered by viral infection or tissue damage during delivery 3. **Thyroid infiltration** by lymphocytes and plasma cells 4. **Release of preformed thyroid hormone** from damaged follicles (thyrotoxic phase) 5. **Subsequent hypothyroidism** as the gland recovers but TPO antibodies persist **Clinical Pearl:** Postpartum thyroiditis is often misdiagnosed as Graves' disease or postpartum depression. The key is the **suppressed uptake** — if uptake is high, think Graves'; if uptake is low, think thyroiditis. ### Natural History **Mnemonic: PHASE** — Postpartum Hyperthyroidism, Autoimmune, Suppressed uptake, Eventually resolves (usually), Sometimes hypothyroid - **Thyrotoxic phase** (weeks 1–4, sometimes extending to 12): Hormone release → mild symptoms - **Hypothyroid phase** (weeks 4–12): Gland exhaustion + antibody-mediated destruction → hypothyroidism - **Recovery phase** (weeks 12–52): Most patients return to euthyroidism; 20–30% develop permanent hypothyroidism ### Management No antithyroid drugs (PTU, methimazole) are needed — the thyrotoxicosis is self-limited. Management is supportive: - Beta-blockers (propranolol) for symptomatic relief - Reassurance about the benign, self-limited nature - Close follow-up of TSH and free T4 to detect hypothyroid phase - Levothyroxine if hypothyroidism develops [cite:Harrison 21e Ch 405]