What is the mechanism of action (MOA) of Armor Thyroid (desiccated thyroid), a medication containing T4 (thyroxine) and T3 (triiodothyronine)?

Mechanism of Action of Armor Thyroid (Desiccated Thyroid)

Armor Thyroid provides exogenous thyroid hormones T4 (thyroxine) and T3 (triiodothyronine) that directly replace deficient endogenous thyroid hormones, with these hormones diffusing into cell nuclei to bind thyroid receptor proteins attached to DNA, activating gene transcription and protein synthesis to produce their physiologic effects. 1

Direct Hormonal Replacement Mechanism

• The primary mechanism is direct supplementation of thyroid hormones rather than stimulation of endogenous production 1
• T3 and T4 from desiccated thyroid diffuse into cell nuclei and bind to thyroid receptor proteins attached to DNA, forming a hormone-nuclear receptor complex that activates gene transcription and synthesis of messenger RNA and cytoplasmic proteins 1
• The physiological actions are produced predominantly by T3, which is the metabolically active form 1

Composition and Hormone Content

• Desiccated thyroid (Armor Thyroid) contains both T4 and T3 in an approximate ratio of 4:1 (T4:T3 molar ratio of 4.3), derived from porcine thyroid glands 2
• One grain of desiccated thyroid contains approximately 12 mcg T3 and 64 mcg T4 2
• The T3 content provides approximately 39% of the thyromimetic activity of desiccated thyroid preparations 2

Pharmacologic Actions

• Both T4 and T3 are absorbed from the gastrointestinal tract, with T3 absorption from desiccated thyroid being bioequivalent to synthetic T3 3
• The exogenous T4 component undergoes peripheral conversion to T3 via type 1 deiodinase (DIO1) in the liver, similar to endogenous T4 metabolism 4
• Approximately 80% of circulating T3 in normal physiology is derived from peripheral T4 deiodination 1

Key Clinical Distinction from Synthetic Levothyroxine

• Unlike levothyroxine monotherapy, desiccated thyroid provides preformed T3, resulting in rapid T3 absorption with peak serum T3 levels occurring within 2 hours 3
• This direct T3 delivery bypasses the need for complete reliance on peripheral T4-to-T3 conversion 4, 5
• The mean daily dose of desiccated thyroid needed to normalize TSH contains approximately 11 mcg T3, though some patients may require higher doses 5

Important Clinical Caveats

• The rapid T3 absorption can cause transient supraphysiologic T3 elevations in the absorption phase, with peak Free T3 Index reaching approximately 500 after T3-containing preparations compared to 290 after T4 alone 3, 6
• These T3 peaks may cause palpitations or other symptoms of transient thyrotoxicosis 7, 6
• Overtreatment manifests as suppressed TSH with hyperthyroid symptoms, requiring dose reduction 4
• Desiccated thyroid remains outside formal FDA oversight, with consistency of T4 and T3 contents monitored only by manufacturers 5