What is Thyroxine (T4) & Why Is it Important for Athletes?

What Is Free T4?

Free T4 (thyroxine) is one of the two main hormones produced by the thyroid gland. It plays a central role in regulating metabolism, energy production, and protein synthesis — all of which are crucial for athletic performance.

Free T4 is the inactive precursor to T3 (the active thyroid hormone). Together, they support energy output, recovery, and endurance.

"Free T4 is one of the key thyroid hormones regulating your metabolic rate. In athletes, altered T4 can be linked to overtraining, low energy availability or thyroid dysfunction. Testing it together with TSH and free T3 provides a more accurate view of thyroid status and how your body is converting and using thyroid hormones."

Dr Thom Phillips

What are normal Thyroxine (T4) levels for athletes?

We looked at blood test data from EDGE customers and found the median T4 levels for women are 15.3 pmol/L and 16.8 pmol/L for men.

Here is a breakdown of Thyroxine (T4) levels by age:

Source: EDGE customer blood test results Apr 2023 – March 2025.

The labs we used to analyse blood samples state a healthy range for Thyroxine (T4) is generally between 12-22 pmol/L.

Why It’s Important for Performance

1. Energy Metabolism

   T4 influences how efficiently the body converts fuel into energy. Low levels can cause sluggishness, weight gain, and fatigue, while excessive levels may lead to muscle breakdown or heart strain.

2. Training Response

   Thyroid hormones regulate training adaptation and metabolic rate. Imbalances can disrupt recovery, immune health, and body composition goals.

Symptoms of Thyroid Dysfunction

Thyroid dysfunction can be subtle but has a powerful impact on energy, metabolism, and physical performance. Common symptoms to watch for include:

• Unexplained fatigue or weakness

• Sensitivity to cold or heat

• Weight gain or loss

• Slow recovery or poor exercise tolerance

• Mood swings or depression

Why Test Free T4?

Free T4 offers crucial insight into how well your thyroid is functioning and whether it’s supporting your training demands. Consider testing:

• When experiencing unexplained fatigue or metabolism shifts

• To screen for thyroid dysfunction alongside TSH and T3

• If performance, recovery, or weight regulation has changed

In Summary

• Free T4 helps control energy production and metabolism

• Low levels = sluggishness and fatigue. High levels = potential overdrive

• Best interpreted with TSH and Free T3