From the FDA Drug Label
The physiological actions of thyroid hormones are produced predominantly by T3, the majority of which (approximately 80%) is derived from T4 by deiodination in peripheral tissues. Oral levothyroxine sodium is a synthetic T4 hormone that exerts the same physiologic effect as endogenous T4, thereby maintaining normal T4 levels when a deficiency is present.
The FDA drug label does not answer the question.
From the Research
The thyroid gland produces approximately 80-100 micrograms (mcg) of thyroxine (T4) daily in healthy adults. This production level is crucial for maintaining proper metabolic function throughout the body, as thyroid hormones influence nearly every organ system, controlling metabolism, heart rate, body temperature, and energy production 1. T4 is the main hormone secreted by the thyroid, accounting for about 80% of its output, while the remaining 20% consists of triiodothyronine (T3) 1.
Key Points About Thyroxine Production
- Most T4 circulates in the bloodstream bound to proteins, with only a small fraction (0.02-0.04%) existing in the free, active form 1.
- The body converts much of this T4 to T3 in peripheral tissues through enzymatic deiodination, as T3 is the more biologically active form 2.
- This conversion occurs primarily in the liver and kidneys.
- T4 production is regulated by thyroid-stimulating hormone (TSH) from the pituitary gland, which responds to feedback from circulating thyroid hormone levels 3.
Regulation and Clinical Significance
The precise regulation of T4 production maintains proper metabolic function throughout the body. Thyroid hormones influence nearly every organ system, making their balance critical for health. The diagnosis and treatment of thyroid disorders, such as hypothyroidism, often involve measuring T4 levels and adjusting treatments accordingly 3, 2.
Evidence Quality and Recommendations
The most recent and highest quality studies support the understanding that T4 production is a critical component of thyroid function and overall health 1. While older studies provide foundational knowledge, the field of endocrinology continues to evolve, with newer research offering insights into the nuances of thyroid hormone production and regulation 2. Therefore, based on the strongest and most recent evidence, the normal production level of thyroxine (T4) by the thyroid gland is approximately 80-100 micrograms (mcg) daily in healthy adults.