A 28-year-old man from Mumbai presents with a 2-month history of palpitations, tremor, heat intolerance, and weight loss of 6 kg despite increased appetite. On examination, he is tachycardic (104 bpm), has a fine tremor, warm moist skin, and a diffuse non-tender goiter. His serum TSH is 0.08 mIU/L (normal 0.4–4.0), free T4 is 28 pmol/L (normal 10–20), and free T3 is 9.2 pmol/L (normal 3.5–7.0). TSI (thyroid-stimulating immunoglobulin) is positive. Which of the following best explains the suppressed TSH in the setting of elevated free T4 and T3?

Explanation

## Thyroid Hormone Negative Feedback in Graves Disease ### The Hyperthyroid Feedback Loop In primary hyperthyroidism (Graves disease), the thyroid gland produces excessive T4 and T3 independent of TSH control. The high circulating free T4 and T3 exert strong negative feedback on both the hypothalamus (inhibiting TRH) and anterior pituitary (inhibiting TSH), resulting in suppressed TSH. ### Mechanism of TSH Suppression **Key Point:** TSH suppression in hyperthyroidism is a **physiologic response** to excess thyroid hormone, not a pathologic failure of the pituitary. The pituitary is functioning normally — it is simply being inhibited by high free T4 and T3 levels. ### Feedback Sites | Site | Hormone | Effect | |------|---------|--------| | Anterior pituitary | Free T4, T3 | ↓ TSH synthesis and secretion | | Hypothalamus | Free T4, T3 | ↓ TRH secretion | | Peripheral tissues | Free T4, T3 | ↑ Metabolic rate, thermogenesis | ### Diagnostic Features of Graves Disease **High-Yield:** The combination of: - Suppressed TSH (<0.1 mIU/L) - Elevated free T4 AND free T3 (both elevated, not just T4) - Positive TSI or TRAb (thyroid receptor antibodies) - Diffuse goiter without nodules - Orbital signs (exophthalmos, lid lag) in some cases confirms **primary hyperthyroidism** with autoimmune etiology. ### Clinical Pearl **Clinical Pearl:** The elevated free T3 is particularly important in Graves disease. TSI-mediated activation of thyroid follicular cells preferentially increases T3 secretion relative to T4, explaining why some patients have T3-dominant hyperthyroidism with marked symptoms despite only mildly elevated T4. ### Why Not the Other Mechanisms? - **Thyrotroph destruction** would cause secondary hypothyroidism (low TSH + low free T4) — not hyperthyroidism. - **Excessive TRH** would increase TSH, not suppress it. - **Impaired T4→T3 conversion in the hypothalamus** is not a physiologic mechanism for TSH suppression; local hypothalamic T3 does contribute to feedback, but the primary effect is systemic free T3 and T4. ### Pathophysiology Flowchart ```mermaid flowchart TD A[TSI binds to TSH receptor on thyroid]:::outcome A --> B[Thyroid follicular cell activation]:::action B --> C[↑ T4 and T3 synthesis and release]:::action C --> D[High circulating free T4 and T3]:::outcome D --> E{Feedback on HPT axis}:::decision E -->|Anterior pituitary| F[↓ TSH synthesis]:::action E -->|Hypothalamus| G[↓ TRH secretion]:::action F --> H[Suppressed serum TSH]:::outcome G --> H ``` [cite:Harrison 21e Ch 397; KD Tripathi 8e Ch 30]