How is T4 converted to T3?

Approximately 80% of circulating T3 is produced by peripheral deiodination of T4, primarily in the liver and kidneys by deiodinase enzymes (DIO1 and DIO2). Only 20% of T3 is secreted directly from the thyroid gland.

Who might need T3 supplementation alongside T4?

Some patients feel poorly on T4 alone despite normal TSH, particularly those with polymorphisms in DIO2 (the enzyme that converts T4 to T3 in peripheral tissues). A small subset may benefit from combined T4/T3 therapy.

What is the T3:T4 potency ratio?

T3 is approximately 3-4 times more potent than T4 on a microgram basis because T3 directly binds thyroid receptors, while T4 must be converted. When switching between preparations, this ratio must be accounted for to avoid under- or over-treatment.

T3/T4 Conversion Rate

Estimate peripheral T4 to T3 conversion: approximately 80% of T3 comes from T4 deiodination. Calculate T3 equivalent: 1 mcg T4 = approximately 0.25-0.33 mcg T3. Useful for thyroid hormone replacement optimization.

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Frequently Asked Questions

How is T4 converted to T3?

Who might need T3 supplementation alongside T4?

What is the T3:T4 potency ratio?