Understanding Lab Tests for TBG Deficiency
Written by Private MD Labs Medical Writing Team • Last Updated: September 8, 2025
Thyroxine Binding Globulin (TBG) deficiency is a rare inherited condition where the body cannot produce adequate amounts of TBG, a protein that transports thyroid hormones in the blood. While this condition typically doesn't cause symptoms because free thyroid hormone levels remain normal, specific lab testing is required for accurate diagnosis.
Recommended Test
TBG (Thyroxine Binding Globulin)
This test directly measures the level of thyroxine binding globulin in the blood. In TBG deficiency, this test will show significantly reduced or absent TBG levels, confirming the diagnosis of this rare inherited condition. The test is essential for distinguishing TBG deficiency from other causes of abnormal thyroid function tests.
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Diagnosing TBG Deficiency
TBG deficiency is often discovered incidentally during routine thyroid function testing when total thyroid hormone levels appear low but the patient has no symptoms of hypothyroidism. A specific TBG test is essential to confirm the diagnosis and distinguish it from other thyroid disorders.
Understanding Test Results
Low or undetectable TBG levels confirm the diagnosis of TBG deficiency. It's important to note that while TBG levels are reduced, free thyroid hormone levels (free T4 and free T3) typically remain within normal ranges, which is why most people with this condition experience no symptoms and require no treatment.
Important Considerations
The information here is for educational purposes only and is not a substitute for professional medical advice. Always consult with a healthcare provider for diagnosis and treatment of thyroid-related conditions.
