T3 to T4 Ratio in Human versus Porcine Thyroid
Human Thyroid Hormone Production
The human thyroid gland produces thyroid hormones in a ratio of approximately 4:1 (T4:T3), with T4 constituting 80% and T3 constituting 20% of direct thyroidal secretion 1.
- The human thyroid directly secretes both T4 (thyroxine) and T3 (triiodothyronine), with T4 representing the predominant hormone at 80% of total thyroidal output 1.
- T3 comprises approximately 20% of direct thyroid gland secretion in humans 1.
- However, in the circulation, approximately 72% of circulating T3 is produced by peripheral conversion of T4 to T3 through 5'-monodeiodination in peripheral tissues, with only 28% deriving from direct thyroidal secretion 2.
- The body's overall T4:T3 secretion ratio is often cited as 11:1 when considering the total physiologic production and conversion 3.
Porcine (Desiccated Thyroid) T3 to T4 Ratio
Porcine thyroid preparations contain T4 and T3 in a ratio of approximately 4.22:1 (T4:T3), which differs substantially from the human physiologic ratio 3.
- Commercial desiccated thyroid products such as Armour Thyroid and Nature-Throid, derived from porcine thyroid glands, contain a T4:T3 ratio of 4.22:1 3.
- This 4.22:1 ratio in porcine preparations provides proportionally more T3 relative to T4 compared to what the human thyroid naturally produces when considering the total physiologic T4:T3 ratio of 11:1 3.
Clinical Implications of the Ratio Difference
- The higher T3 content in porcine thyroid preparations (4.22:1) compared to the human physiologic ratio (11:1) may result in supraphysiologic T3 levels and increased risk of iatrogenic hyperthyroidism 3.
- Patients treated with levothyroxine (T4) alone typically achieve a free T3 to free T4 ratio that is lower than in euthyroid individuals, as peripheral conversion produces most circulating T3 4.
- The short half-life of T3 in porcine preparations necessitates consideration of divided dosing or sustained-release formulations to maintain stable hormone levels throughout the day 3.
Important Caveats
- The conversion of T4 to T3 can be influenced by various clinical conditions, with sick euthyroid patients showing significantly reduced conversion rates (52.5% versus 72% in healthy controls) 2.
- In treated hypothyroidism, whether central or primary, patients demonstrate a lower free T3 to free T4 ratio compared to euthyroid controls, with higher free T4 concentrations required to maintain adequate T3 levels 4.