Free T3: The crucial thyroid hormone your doctor may not be testing

In the United States, standard thyroid function testing typically includes measuring Thyroid Stimulating Hormone (TSH) and Free Thyroxine (FT4) levels. However, Free Triiodothyronine (FT3) testing is not routine and is rarely prescribed.

Understanding T3 and how it differs from T4

Thyroid hormones – Thyroxine (T4) and Triiodothyronine (T3) – have different biological functions, and their ratio is crucial for overall well-being.

Thyroxine (T4) is the precursor to T3 and acts as the body's reserve form of thyroid hormones. On its own, T4 has limited activity and must first be converted into its active form – Triiodothyronine (T3).

T3 is the primary active thyroid hormone that directly influences metabolism, brain function, cardiovascular system, and other vital functions. It is approximately 3-5 times more potent than T4, though its levels in the body are significantly lower – about 10-20% of total thyroid hormones. Meanwhile, T4 makes up 80-90% of all thyroid hormones but primarily serves as a reserve.

Additionally, T3 easily enters cells and binds to receptors, activating metabolic processes, whereas T4 cannot directly affect cells and must first be converted to T3.

Therefore, while T4 is the main hormone circulating in the blood, T3 is responsible for all key physiological effects, and its deficiency can lead to hypothyroid symptoms even when TSH and T4 levels are within normal ranges.

Key functions of T3 in the body

1. Metabolism and energy production

• Controls metabolic rate
• Regulates cellular energy production
• Influences how the body uses nutrients

2. Brain function and mental health

• Supports cognitive function
• Regulates nervous system activity
• Helps manage stress response

3. Cardiovascular health

• Influences heart rate and strength
• Affects blood vessel function
• Supports healthy circulation

4. Weight management

• Controls fat metabolism
• Influences basal metabolic rate
• Helps regulate body weight

5. Digestive health

• Supports gut function
• Influences nutrient absorption
• Affects digestive motility

6. Hormonal balance

• Supports reproductive health
• Influences other hormone systems
• Helps maintain endocrine balance

Conclusion: T3 is the primary active hormone affecting metabolism, brain function, heart health, weight, and overall well-being. Although standard testing typically focuses on TSH and T4, without evaluating T3, it's impossible to understand how effectively the body is utilizing thyroid hormones. Blood tests may show adequate hormone levels, but the body might not be using them efficiently.Therefore, if a patient shows any symptoms of hypothyroidism but standard tests are normal or borderline, it's important to check Free T3 levels to identify conversion issues and determine the right approach to recovery.