A 28-year-old woman presents with a 3-month history of progressive fatigue, weight gain of 8 kg, cold intolerance, and dry skin. On examination, she has a regular pulse of 58 bpm, delayed ankle reflexes, and a diffuse, non-tender thyroid enlargement. Laboratory investigations show: TSH 18.5 mIU/L (normal 0.4–4.0), free T4 0.8 ng/dL (normal 0.8–1.8), free T3 2.1 pg/mL (normal 2.3–4.2). Thyroid peroxidase (TPO) antibodies are positive at 450 IU/mL. What is the most appropriate immediate next step in management?

Explanation

## Clinical Diagnosis This patient presents with classic features of **primary hypothyroidism secondary to autoimmune thyroiditis (Hashimoto's disease)**: - Elevated TSH with low-normal to low free T4 - Positive TPO antibodies (diagnostic) - Diffuse thyroid enlargement (goiter) - Hypothyroid symptoms (fatigue, weight gain, cold intolerance, bradycardia, delayed reflexes) ## Management Algorithm ```mermaid flowchart TD A[Confirmed primary hypothyroidism<br/>TSH elevated, free T4 low]:::outcome --> B{Symptomatic?}:::decision B -->|Yes| C[Initiate levothyroxine<br/>25–50 mcg/day]:::action B -->|No| D[Observation + repeat TSH<br/>in 4–6 weeks] C --> E[Recheck TSH at 6–8 weeks]:::action E --> F[Titrate dose by 25–50 mcg<br/>until TSH normal]:::action F --> G[Maintenance: annual TSH monitoring]:::action ``` ## Key Point: **Levothyroxine replacement is the definitive treatment for primary hypothyroidism.** The diagnosis is already confirmed by: - Elevated TSH (18.5 mIU/L) - Low-normal free T4 (0.8 ng/dL, at lower limit) - Positive TPO antibodies - Clinical hypothyroid symptoms **High-Yield:** Starting dose in non-pregnant, non-cardiac adults is **25–50 mcg/day**. In elderly or cardiac patients, start lower (12.5–25 mcg/day). Dose is increased by 25–50 mcg every 6–8 weeks based on TSH response. ## Why Levothyroxine Now? 1. **Symptomatic hypothyroidism** — patient has fatigue, weight gain, bradycardia, delayed reflexes 2. **Confirmed diagnosis** — TPO+ autoimmune thyroiditis with biochemical confirmation 3. **No contraindications** — no acute cardiac ischemia, no pregnancy mentioned 4. **Standard of care** — levothyroxine is the first-line replacement hormone [cite:Harrison 21e Ch 405] ## Clinical Pearl: Levothyroxine should be taken on an empty stomach (30–60 min before food) for optimal absorption. Absorption is impaired by iron, calcium, and proton pump inhibitors — counsel the patient accordingly. ## Monitoring Schedule | Timepoint | Action | |-----------|--------| | Baseline | Start levothyroxine 25–50 mcg/day | | 6–8 weeks | Check TSH; adjust dose if needed | | 6–8 weeks after each dose change | Repeat TSH | | Once stable | Annual TSH monitoring | | Target TSH | 0.5–2.5 mIU/L (individualize for age, cardiac status) |