## Negative Feedback Suppression of TSH in Hyperthyroidism **Key Point:** In hyperthyroidism, excess circulating T3 and T4 suppress TSH secretion through enhanced negative feedback at both the hypothalamic (TRH) and pituitary (TSH) levels, resulting in a suppressed TSH despite elevated thyroid hormones. ### Normal Negative Feedback Mechanism Thyroid hormones (especially T3) inhibit: 1. **Hypothalamus:** Suppress TRH (thyrotropin-releasing hormone) synthesis and release 2. **Anterior Pituitary:** Suppress TSH synthesis and secretion 3. **Pituitary Thyrotroph Cells:** Reduce TSH β-subunit gene expression and TSH receptor sensitivity ### In This Patient's Case: Graves' Disease The clinical and biochemical findings indicate **Graves' disease** (autoimmune hyperthyroidism): - Positive TSI (thyroid-stimulating immunoglobulin / TRAb) - Diffuse goitre with bruit (increased vascularity) - Suppressed TSH with elevated free T4 and T3 - Classic adrenergic symptoms (palpitations, tremor, heat intolerance) **High-Yield:** TSI antibodies bind to TSH receptors on thyroid cells and activate them, causing uncontrolled thyroid hormone synthesis and release *independent of TSH*. However, the excess T3 and T4 produced still exert negative feedback on the pituitary and hypothalamus, suppressing TSH. ### Feedback Suppression in Graves' Disease ```mermaid flowchart TD A[TSI antibodies bind TSH receptor]:::outcome --> B[Thyroid activation independent of TSH]:::action B --> C[↑ T4 and T3 synthesis and secretion]:::action C --> D[Elevated circulating T3 and T4]:::outcome D --> E{Negative feedback on pituitary/hypothalamus?}:::decision E -->|Yes| F[↓ TRH secretion]:::action F --> G[↓ TSH secretion]:::action G --> H[Suppressed TSH despite high T4/T3]:::outcome I[TSI-driven thyroid activation continues] -.->|Independent of TSH| C ``` **Clinical Pearl:** The key distinction in Graves' disease is that thyroid hormone production is driven by TSI antibodies (not TSH), but the resulting excess T3 and T4 still suppress TSH through normal negative feedback. This creates the paradoxical pattern of **suppressed TSH with elevated thyroid hormones** — the opposite of primary hypothyroidism. ### Biochemical Patterns: Hyperthyroidism vs. Hypothyroidism | Parameter | Graves' Disease (Hyperthyroidism) | Hashimoto's (Hypothyroidism) | Normal | |-----------|-----------------------------------|------------------------------|--------| | TSH | ↓ (suppressed, <0.1) | ↑↑ (>10) | 0.5–5 mIU/L | | Free T4 | ↑ | ↓ | 10–20 pmol/L | | Free T3 | ↑ | ↓ | 3.5–7.8 pmol/L | | TSI/TRAb | Positive | Negative | Negative | | Anti-TPO | May be present | Positive | Negative | | Mechanism | Antibody-driven activation | Autoimmune destruction | Normal regulation | ### T3 as the Primary Negative Feedback Agent **Mnemonic:** **T3-FEEDBACK** — T3 is the most potent thyroid hormone for negative feedback, Feedback occurs at hypothalamus and pituitary, Excess T3 suppresses TRH and TSH, Elevated free T3 is the strongest inhibitor, Deiodinase converts T4 to T3 peripherally, Back-regulation prevents thyroid overstimulation, Antibodies (TSI) bypass TSH but not feedback **High-Yield:** T3 is approximately 3–4 times more potent than T4 at suppressing TSH. In Graves' disease, the elevated free T3 (often disproportionately elevated relative to T4) is the primary driver of TSH suppression. [cite:Harrison 21e Ch 405; Guyton & Hall 14e Ch 84]
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