A 28-year-old man with newly diagnosed Graves' disease is being investigated. Which of the following findings is NOT expected in active hyperthyroidism?

Explanation

## Laboratory Diagnosis of Hyperthyroidism **Key Point:** The hallmark of primary hyperthyroidism is **suppressed TSH** (typically <0.1 mIU/L) with elevated free T4 and/or free T3. This reflects the negative feedback suppression of the pituitary by excess thyroid hormones. ### Expected Laboratory Findings in Graves' Disease | Finding | Expected Result | Interpretation | |---------|-----------------|----------------| | Serum TSH | Suppressed (<0.1 mIU/L) | Loss of negative feedback | | Free T4 | Elevated (>25 pmol/L or >2.0 ng/dL) | Primary thyroid overproduction | | Free T3 | Elevated (>7.5 pmol/L or >5.0 pg/dL) | T3 toxicosis | | TSH receptor antibodies (TRAb) | Positive | Pathognomonic for Graves' disease | | Radioactive iodine uptake (RAIU) | Elevated (>30% at 24 hrs) | Increased thyroid hormone synthesis | **High-Yield:** In Graves' disease, TSH is suppressed BECAUSE thyroid hormone levels are high. The pituitary responds normally to the negative feedback — it is the thyroid gland that is overactive. ### Why Option 4 is Wrong Elevated TSH with low free T4 is the pattern of **primary hypothyroidism**, not hyperthyroidism. This occurs when: - The thyroid gland fails to produce adequate hormone - The pituitary responds by increasing TSH to try to stimulate the failing gland - This is the opposite of the feedback mechanism in hyperthyroidism The presence of goitre does not change this principle — whether the goitre is due to autoimmune thyroiditis (Hashimoto's) or iodine deficiency, elevated TSH + low T4 = hypothyroidism. **Mnemonic:** **SHALT** = Suppressed TSH, High T4, Antibodies (TRAb), Low TSH = Thyroid overactive ### Distinguishing Graves' from Other Causes of Hyperthyroidism ```mermaid flowchart TD A[Hyperthyroidism<br/>Suppressed TSH + High T4/T3]:::outcome --> B{RAIU elevated?}:::decision B -->|Yes| C{TRAb positive?}:::decision C -->|Yes| D[Graves' disease]:::outcome C -->|No| E[Toxic multinodular goitre<br/>or toxic adenoma]:::outcome B -->|No| F{Thyroid pain/tenderness?}:::decision F -->|Yes| G[Thyroiditis<br/>Subacute or postpartum]:::outcome F -->|No| H[Factitious hyperthyroidism<br/>or iodine-induced]:::outcome ``` **Clinical Pearl:** TRAb (TSH receptor antibodies) are specific for Graves' disease and help differentiate it from other causes of hyperthyroidism. They can also predict neonatal thyrotoxicosis if present in a pregnant woman.