Overview

Thyroxine-Binding Globulin (TBG) is a transport protein that carries thyroid hormones (mainly T4 and T3) through your bloodstream. These hormones control your metabolism - how your body uses energy. Since thyroid hormones are not very water-soluble, they need a “carrier” to travel around the body. That’s where TBG comes in. Although TBG itself doesn’t have hormonal activity, changes in its levels can affect how much free (active) thyroid hormone is available to your cells. Understanding your TBG level gives more context to your thyroid health, especially if other thyroid test results (like TSH or T4) seem inconsistent.

Testing Levels:

• High ( ) - High TBG means your body has more “carriers” for thyroid hormones, which may lead to more total thyroid hormone in the blood. However, this doesn’t necessarily mean your thyroid is overactive - free thyroid hormone levels may still be normal. High TBG is commonly seen during pregnancy, when taking oestrogen-containing medications, or due to genetic variations. It's usually not a problem unless free hormone levels are also affected.
  
  
• Mid - A normal TBG result means your body has the right amount of transport proteins for thyroid hormones. This usually suggests that your thyroid hormone levels are being regulated effectively and your liver is functioning well.
  
  Slightly elevated TBG may reflect mild hormonal changes or the influence of oestrogen-based medications. It's generally not concerning on its own but should be interpreted alongside free T4 and TSH to confirm that your thyroid is functioning properly.
  
  Slightly low TBG could be due to medications, liver function, or low oestrogen levels. It may not mean anything serious on its own, but it’s important to ensure your free thyroid hormone levels are normal too.
  
  
• Low ( ) - Low TBG means fewer transport proteins are available, which might result in low total thyroid hormone levels - even if free (active) hormones are normal. This can occur in liver disease, malnutrition, androgen therapy, or due to inherited conditions. On its own, low TBG isn’t always a problem - but it can cause confusion if thyroid tests aren’t interpreted correctly.

Why We Test Thyroxine-Binding Globulin (TBG)

TBG testing is helpful when thyroid hormone levels seem unusual or inconsistent, especially when total T4 or T3 levels are abnormal but free hormone and TSH levels don’t match the picture. It’s often used when evaluating thyroid function during pregnancy, when on hormone therapy, or when looking into inherited thyroid-related conditions.

What Affects It

TBG levels can change due to hormones, liver function, and genetic factors. Oestrogen (as found in pregnancy or oral contraceptives) increases TBG, while androgens, liver disease, and certain medications (like steroids) can lower it. Rare inherited conditions can also affect how much TBG you produce. It’s important to interpret TBG levels alongside other thyroid tests to get the full picture.

How to Improve

If your TBG is high due to oestrogen (e.g. during pregnancy or HRT), no action is usually needed unless thyroid symptoms appear. If TBG is low, your doctor may review any medications you’re taking, check liver function, or test for underlying conditions. Because TBG levels can't easily be changed through diet or lifestyle, the focus is usually on managing underlying causes and ensuring that your active (free) thyroid hormone levels are balanced.

Common Name: Thyroxine-Binding Globulin (Serum)