# Thyroxine (T4), Thyroxine-binding Globulin (TBG), and T4−TBG Index

> This test measures the total amount of thyroxine (T4) in your blood and the protein that carries it, called thyroxine-binding globulin (TBG). By comparing these levels, the test calculates an index that estimates how much thyroid hormone is actually available for your body to use. This is particularly useful for checking thyroid function in situations that affect blood proteins, such as pregnancy or hormone therapy, where standard tests might be misleading. It helps determine if abnormal hormone levels are due to a thyroid problem or simply a change in carrier proteins. This test does not diagnose the specific underlying cause of thyroid disease or replace the need for measuring thyroid-stimulating hormone (TSH).

- Labcorp code: 004457
- Category: [Thyroid](https://app.picardo.health/labs/tests/thyroid.md)
- Price: shown at checkout, before payment.
- Blood draw: free at a Labcorp location, or $75.00 at home.
- Order link: https://app.picardo.health/labs/order?tests=004457&ref=assistant

## About ordering

A licensed physician reviews every order before any test is run, and results arrive in the person's Picardo account. Ordering is open to adults 18 and over in the US, except New York, New Jersey, Rhode Island, Pennsylvania, Maryland, Massachusetts, Hawaii and Maine. General health information, not medical advice.