Hormone & Endocrine - Bloody Good
Thyroid

Thyroxine-Binding Globulin (TBG)

Understand your thyroid hormone transport with our TBG Blood Test. This protein carries thyroid hormones through your bloodstream, and measuring its levels helps explain unusual thyroid function test results and guide accurate thyroid health assessment.

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Thyroxine-Binding Globulin (TBG)

Confused by conflicting thyroid test results, or wondering if unusual hormone levels might be related to the proteins that transport hormones in your blood?

Overview

Think of TBG (Thyroid Binding Globulin) as your thyroid hormones' personal chauffeur service, responsible for safely transporting T4 and T3 hormones throughout your bloodstream. This protective carrier binds to about 75% of your circulating thyroid hormones, preventing them from breaking down too quickly while ensuring they're delivered exactly where your body needs them. Just like having too many or too few drivers affects how efficiently people get around a city, abnormal TBG levels can significantly impact how thyroid hormones circulate in your body, even when your thyroid gland is working perfectly fine.

This test becomes particularly valuable when your standard thyroid function tests show confusing results that don't match how you're actually feeling. When TBG levels are high, more thyroid hormone gets "tied up" in transport, making your total hormone levels appear elevated even though the amount of active hormone remains normal—and vice versa when TBG is low. Various factors like liver problems, certain medications, pregnancy, or even genetic variations can affect your TBG levels, so understanding your TBG status helps your doctor accurately interpret your other thyroid tests and determine whether any apparent abnormalities reflect true thyroid issues or simply changes in your body's hormone transport system.

Reviewed by Dr. Vu Tran
Bloody Good’s Chief Medical Officer

Symptoms

You may benefit from this test if you experience:
Feeling tired all the time Always feeling cold even in warm weather Hair falling out in clumps Difficulty losing weight despite diet and exercise

Purpose

Important for investigating discrepancies between thyroid function tests and symptoms. Essential for people with liver disease, kidney problems, or taking medications that affect protein levels. Valuable for interpreting thyroid tests during pregnancy, illness, or when using hormone medications.

How to prepare

No fasting required. Morning collection recommended for consistency with other thyroid tests. Avoid biotin supplements for 3 days before testing. Inform your healthcare provider about pregnancy, liver/kidney conditions, medications, and nutritional status as these significantly affect TBG levels.

Understanding results

Normal range: 12-28 mg/L (varies by laboratory). High TBG: May be caused by pregnancy, estrogen therapy, liver disease, or genetic factors. Low TBG: Often due to liver disease, kidney disease, malnutrition, certain medications, or genetic deficiency. Results help interpret total T4 and T3 levels more accurately.

After the test

Discuss results with your healthcare provider to understand how TBG levels affect interpretation of other thyroid tests. Abnormal TBG may require adjustment of thyroid hormone replacement dosing or switching to monitoring free hormone levels instead of total levels. Address underlying conditions affecting TBG such as liver disease or malnutrition.

Frequently Asked Questions

  • While TBG doesn't directly affect thyroid function, it significantly influences total thyroid hormone measurements. Understanding TBG levels prevents misinterpretation of thyroid tests and inappropriate treatment.
  • Common causes include pregnancy, estrogen therapy, oral contraceptives, liver disease, and genetic variations. These conditions increase TBG production, leading to higher total thyroid hormone levels.
  • TBG deficiency itself doesn't cause symptoms because free thyroid hormone levels remain normal. However, it can make total thyroid hormone levels appear falsely low, potentially leading to unnecessary treatment.
  • Pregnancy typically doubles TBG levels due to increased estrogen, causing total T4 levels to rise significantly while free T4 levels remain normal. This is why free hormone tests are preferred during pregnancy.
  • Medication dosing should be based on free hormone levels and symptoms rather than total hormone levels, especially when TBG is abnormal. TBG measurement helps explain why total levels might be misleading.
  • Yes, TBG deficiency can be inherited and is more common in males due to X-linked inheritance. Family members should be tested if genetic TBG abnormalities are suspected.
Thyroxine-Binding Globulin (TBG)

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