NP Thyroid Contains Both T4 and T3
NP Thyroid is a natural desiccated thyroid preparation derived from porcine thyroid glands that contains both T4 (thyroxine) and T3 (triiodothyronine) in a fixed ratio of approximately 4.22:1 (T4:T3). 1, 2
Composition and Mechanism
Natural desiccated thyroid preparations like NP Thyroid are derived from domesticated animal thyroid glands (porcine) and contain both thyroid hormones as they naturally exist in animal thyroid tissue. 1, 3
The T4:T3 ratio in these preparations (approximately 4.22:1) differs significantly from the human thyroid's natural secretion ratio of approximately 11:1, meaning desiccated thyroid products provide proportionally more T3 than the human body naturally produces. 2
In contrast to synthetic levothyroxine (T4-only) therapy, desiccated thyroid delivers pre-formed T3 directly, bypassing the need for peripheral conversion of T4 to T3 by deiodinase enzymes. 4, 1
Clinical Relevance of Dual Hormone Content
T3 is the metabolically active form of thyroid hormone that binds to nuclear receptors to initiate hormonal effects, while T4 functions primarily as a prohormone that must be converted to T3 for biological activity. 5, 1
Under normal physiology, the liver is the primary site where T4 converts to T3 through type 1 deiodinase (DIO1), a selenium-dependent enzyme that produces the majority of circulating T3. 4
The body prioritizes maintaining stable plasma T3 levels as a biological imperative, with less than 20% of circulating T3 coming directly from thyroid secretion and over 80% from peripheral T4 conversion. 6
Important Caveats
The fixed T4:T3 ratio in desiccated thyroid preparations cannot be adjusted independently, which may lead to supraphysiologic T3 levels when dosed to normalize TSH, particularly given the shorter half-life of T3 compared to T4. 2, 3
Historical use of natural thyroid preparations was associated with thyrotoxic adverse effects when doses were adjusted based on symptom resolution rather than biochemical monitoring. 3
Factors affecting T4 to T3 conversion—including selenium deficiency, iron deficiency, and certain disease states—may theoretically favor preparations containing pre-formed T3, though this remains an area of ongoing investigation. 5, 4