A 52-year-old woman from rural Maharashtra presents with progressive fatigue, weight gain of 8 kg over 6 months, and constipation. She reports cold intolerance and has noticed thinning of eyebrows. On examination, her pulse is 52/min, blood pressure 138/88 mmHg, skin is dry and pale, and there is a firm, 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), and anti-TPO antibodies 320 IU/mL (normal <35). What is the most likely diagnosis?

Explanation

## Clinical Diagnosis: Hashimoto Thyroiditis with Hypothyroidism ### Key Clinical Features **Key Point:** This patient presents with the classic triad of hypothyroidism: elevated TSH, low-normal/low free T4, and positive anti-TPO antibodies—diagnostic of autoimmune thyroiditis (Hashimoto disease). **Clinical Pearl:** The combination of: - Goiter (firm, non-tender thyroid enlargement) - Hypothyroid symptoms (fatigue, weight gain, cold intolerance, constipation) - Bradycardia (52/min) - Dry skin and loss of lateral eyebrows (sign of hypothyroidism) - Elevated TSH with low-normal free T4 points definitively to primary hypothyroidism with autoimmune etiology. ### Laboratory Interpretation | Parameter | Patient Value | Normal Range | Interpretation | |-----------|---------------|--------------|----------------| | TSH | 18.5 mIU/L | 0.4–4.0 | Markedly elevated | | Free T4 | 0.8 ng/dL | 0.8–1.8 | Low-normal (at threshold) | | Anti-TPO | 320 IU/mL | <35 | Strongly positive | **High-Yield:** Anti-TPO antibodies are present in >90% of Hashimoto thyroiditis cases. Their presence confirms autoimmune destruction of the thyroid. ### Pathophysiology of Hashimoto Thyroiditis 1. T-cell and B-cell mediated autoimmune attack on thyroid peroxidase and thyroglobulin 2. Progressive lymphocytic infiltration → thyroid fibrosis 3. Loss of thyroid hormone production → compensatory TSH rise 4. Eventually TSH cannot maintain adequate T4 synthesis → overt hypothyroidism **Key Point:** In early Hashimoto disease, TSH rises *before* free T4 falls—this is called **subclinical hypothyroidism**. This patient is in the **overt hypothyroidism** stage (low-normal T4 + high TSH + symptoms). ### Why Goiter Occurs in Hashimoto Disease TSH stimulation of the damaged gland (attempting to compensate) → thyroid enlargement. Goiter in hypothyroidism is a sign of ongoing TSH stimulation despite autoimmune destruction. ### Treatment Approach **Clinical Pearl:** Levothyroxine replacement is the standard of care. Dose titration is guided by TSH normalization (target TSH 0.5–2.5 mIU/L in most patients). This patient would typically start at 50–75 μg daily, with reassessment at 6–8 weeks. [cite:Harrison 21e Ch 405]