T4

T4 plays a crucial role in regulating metabolism, growth, development, and other bodily functions. It does this by: Increasing the body's energy expenditure, stimulating cell growth and differentiation, maintaining body temperature, influencing heart rate and blood pressure, and affecting brain function and mood. 

T4 levels in the blood can be measured through a blood test called a thyroid function test. This test can help diagnose thyroid disorders, such as hypothyroidism (low T4 levels) or hyperthyroidism (high T4 levels).