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 good appetite. He has anxiety and insomnia. On examination, he is tachycardic (110 bpm at rest), has warm moist skin, fine tremor, and lid lag. Thyroid function tests show TSH 0.05 mIU/L (normal 0.4–4.0), free T4 18.5 pmol/L (normal 10–20), and free T3 8.2 pmol/L (normal 3.5–7.0). A thyroid uptake scan shows uniformly increased uptake throughout the gland. Which of the following best explains the elevated free T3 level relative to free T4 in this patient?

Explanation

## Thyroid Hormone Synthesis and Secretion in Graves' Disease **Key Point:** In primary hyperthyroidism (particularly Graves' disease, as suggested by the uniformly increased uptake), the hyperactive thyroid gland preferentially synthesizes and secretes **T3 over T4** in a ratio that is skewed compared to normal thyroid physiology. ### Normal Thyroid Hormone Secretion | Parameter | Normal Gland | Hyperthyroid Gland | |-----------|--------------|--------------------| | T4 secretion | ~80–100 nmol/day | Markedly increased | | T3 secretion | ~30 nmol/day | Disproportionately increased | | T4:T3 ratio | 10–15:1 | 3–5:1 | | Relative increase in T3 | Baseline | **Preferential** | ### Mechanism of Preferential T3 Synthesis 1. **Increased thyroid peroxidase (TPO) activity** → enhanced iodination of thyroglobulin 2. **Increased coupling enzyme activity** → more T3 and T4 formed from iodinated tyrosines 3. **Preferential coupling to form T3** → when iodine availability is high and enzyme activity is maximal, the coupling reaction favors T3 formation (monoiodotyrosine + diiodotyrosine → T3) over T4 formation (diiodotyrosine + diiodotyrosine → T4) 4. **Increased proteolysis of thyroglobulin** → rapid release of newly synthesized hormones, enriched in T3 **Clinical Pearl:** The **T3:T4 ratio is inverted in hyperthyroidism** — T3 rises disproportionately. This is why some patients present with "T3 thyrotoxicosis" (elevated T3, normal or low-normal T4), especially in the early phases of Graves' disease. **High-Yield:** The uniformly increased uptake on thyroid scan rules out thyroiditis (which shows low uptake) and toxic nodule (which shows focal uptake). This pattern is diagnostic of Graves' disease, where the entire gland is stimulated by TSI (thyroid-stimulating immunoglobulin). ### Why This Matters Clinically - T3 is **4–5 times more potent** than T4 at the tissue level - Preferential T3 secretion in hyperthyroidism explains the **severity of symptoms** (palpitations, tremor, anxiety) despite only modest elevation of free T4 - Some patients may have **isolated T3 elevation** early in disease, making TSH suppression the most sensitive initial screening test [cite:Harrison 21e Ch 405]