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Thyroid Hormones

Thyroid Hormones T4 =>Active T3 Conversion

All the thyroid hormone metabolic activity comes from T3 - T4 is the body's storage form ready to be converted to its active T3  form (Except during pregnancy, when T4 is needed for the fetus to develop properly).

Only ~20% of the body's T3 comes from the synthesis of T3 within the thyroid - the rest of the T3 is derived when the deiodinase enzyme removes one iodine molecule from T4. (the "3" and "4" refer to the number of iodine atoms in each thyroid hormone molecule).

Most T4 →T3 conversion (via deiodinase enzymes) takes place in the liver - but some also in muscles and other peripheral organs, including thyroid, kidneys, spleen and the brain's hypothalamus. The immediate effects of T3 last for 1-2 days.

Some people have a problem converting T4 to T3 not always indicated by lab tests.

Stress-activated hormones interfere with T4 to T3 conversion – stress hormones, such as CORTISOL and EPINEPHRINE (ADRENALINE) can divert the conversion of T4 hormones into inactive forms that do not activate metabolism.

Stress Affects TH production and T4 to T3 Conversion

Result of Deficient T3

Without enough thyroid hormone in its active T3 state, the metabolism slows down - possibly resulting in:

Lowered pulse rate


Goiter (thyroid enlargement)

Weight gain / slow metabolism


Fluid retention

Dry/flaky skin

Cold hands or feet

Hair loss, dry/brittle hair

Brittle nails

Puffy face in morning


Swollen/droopy eyes

Low sex drive

Brain damage

Muscle aches/cramps

PMS, painful periods


Lower IQ

Heart palpitations


Yellow discoloration skin/palms

High cholesterol


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