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Alpha subunit (ASU) of TSH
📖 About
- 🧬 The Alpha Subunit (ASU) of TSH is measured by endocrinologists when investigating unexplained high TSH levels.
- TSH, LH, FSH, and hCG are all glycoproteins composed of two subunits (α and β).
- The β-subunits are unique and provide each hormone’s biological and immunological specificity, while the α-subunit is common to all four.
🩺 Differential Diagnosis
- 📌 TSH-secreting pituitary adenomas – rare (≈1% of pituitary tumours). Some macroadenomas secrete only ASU, making the test diagnostically useful.
- 🤰 Physiological increase in pregnancy – free α-subunit levels rise naturally.
- 🧬 Thyroid Hormone Resistance (RTH) – paradoxical findings of elevated TSH with high FT4.
🧾 Sample Requirements
- For adults: collect blood into a 6 mL red-top tube (plain serum).
⚠️ Conditions Associated with Elevated ASU
- 🧠 Pituitary macroadenoma secreting TSH or ASU alone.
- 👨⚕️ Testicular cancer.
- 🍽️ Endocrine gastroenteropancreatic tumours, e.g., Zollinger–Ellison syndrome.
- 🤰 Abnormal pregnancy states, including gestational trophoblastic disease and some non-trophoblastic malignancies.
🌟 Clinical Pearl
- When interpreting results, remember: an isolated rise in ASU in the setting of high TSH and FT4 strongly suggests a pituitary adenoma rather than thyroid disease.
- ASU testing is highly specialist – usually guided by an endocrinologist to distinguish TSH-secreting adenoma from thyroid hormone resistance.
| The content on this website is provided for educational and informational purposes only to support exam preparation (e.g., MLA, MRCP, USMLE) and learning. This is NOT medical advice, diagnosis, treatment, or professional guidance. It does not replace consultation with a qualified healthcare professional, official guidelines (e.g., NICE, GMC, BNF), or supervised clinical practice. Always verify information with current, authoritative sources. Makindo and its contributors accept no liability for any reliance on this content, including errors, omissions, or any resulting harm, loss, or consequences. By using this site, you agree to these terms. |
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