A 28-year-old woman from Mumbai presents with a 3-month history of progressive fatigue, weight gain of 8 kg, cold intolerance, and dry skin. On examination, she has a non-tender, diffuse goiter and delayed relaxation of deep tendon reflexes. Serum TSH is 18.5 mIU/L (normal 0.4–4.0), free T₄ is 6.2 pmol/L (normal 9–19), and anti-TPO antibodies are positive. Which of the following best explains the elevated TSH despite low free T₄ in this patient?

Explanation

## Pathophysiology of TSH Elevation in Primary Hypothyroidism ### The Hypothalamic-Pituitary-Thyroid (HPT) Axis The HPT axis operates via negative feedback. When thyroid hormone (free T₄ and T₃) levels fall, the inhibitory effect on the anterior pituitary is removed, leading to compensatory TSH release. ### Mechanism in This Patient **Key Point:** In primary hypothyroidism (thyroid gland failure), TSH rises because the pituitary is attempting to stimulate an unresponsive thyroid gland. The low free T₄ removes the normal negative feedback restraint on TSH secretion. 1. Thyroid autoimmunity (anti-TPO positive) → destruction of thyroid follicles 2. Reduced T₄ synthesis and secretion 3. Decreased circulating free T₄ 4. Loss of negative feedback at the anterior pituitary and hypothalamus 5. **Increased TRH release** (from hypothalamus) and **increased TSH release** (from anterior pituitary) 6. TSH rises in an attempt to stimulate the failing thyroid ### Clinical Correlation **Clinical Pearl:** The elevated TSH with low free T₄ is the hallmark of **primary hypothyroidism** and confirms that the pituitary and hypothalamus are functioning normally — they are responding appropriately to the low thyroid hormone signal. ### Diagnostic Interpretation | Parameter | Primary Hypothyroidism | Secondary Hypothyroidism | Tertiary Hypothyroidism | | --- | --- | --- | --- | | Free T₄ | ↓ | ↓ | ↓ | | TSH | ↑↑ | ↓ | ↓ | | TRH | ↑ | ↓ | ↑ | | Site of lesion | Thyroid gland | Anterior pituitary | Hypothalamus | **High-Yield:** The TSH level is the most sensitive marker of thyroid status in primary hypothyroidism. A TSH >10 mIU/L with low free T₄ confirms overt primary hypothyroidism and is an indication for levothyroxine replacement. ### Why This Patient Has Autoimmune Thyroiditis - Anti-TPO antibodies indicate Hashimoto thyroiditis - Diffuse goiter, positive antibodies, and elevated TSH are classic features - Female predominance (10:1 ratio vs. males) - Most common cause of hypothyroidism in iodine-sufficient regions [cite:Harrison 21e Ch 397]