Difference between T3 and T4?

[Guest guest]

Can someone please tell me the difference between T 4 and T3? Does a normal

T4 level indicate the need to test for T3?

Thanks

Manoj

[Guest guest]

In a message dated 01/01/2002 8:17:09 AM Pacific Standard Time, manoj@... writes:

Can someone please tell me the difference between T 4 and T3? Does a normal

T4 level indicate the need to test for T3?

Manoj,

Here is an explanation from "Doctor's Manual for 's Thyroid Syndrome." I tried to omit some of the technical stuff concerning precisely how T4 is converted to T3 and RT3 but if you want more, I can quote the whole section. I hope this answers your question.

"The hypothalamus stimulates the pituitary gland to produce Thyroid Stimulating Hormone (TSH). TSH stimulates the thyroid gland to produce T4 (thyroxine) which is the raw material or precursor used to make the active thyroid hormone T3. T4 is also converted to Reverse T3 which is physiologically inactive.

"T4 is not the physiologically active hormone and needs to be converted by an enzyme called 5'-deiodinase to become the active hormone T3, or liothyronine. At least 84% of the body's daily production of T3 is produced in this way, not in the

thyroid gland but in the tissues of the body (liver, kidney, etc.).

"T3 is the physiologically active thyroid hormone and is four times more potent, and three times shorter acting than T4. T3 is what goes to all the cells in the body and tells them how fast to operate and that determines the body temperature. 5' deiodinase has two jobs. It converts T4 to T3 and it converts RT3 to T2. 5 deiodinase (as opposed to 5' deiodinase) is the enzyme that converts T4 to RT3.

"T4 accounts for most of what is produced in the thyroid gland itself (about 10 times less T3 and about 30 times less RT3 is produced than T4. Most T4 is converted to RT3 than to T3. The fact that serum RT3 concentrations are lower than those of T3 reflects its more rapid metabolic cleaarance.

"T4 to T3 conversion is inhibited by stress, acute and chronic illness, fasting, glucocorticoids, propylthiouracil, and other things. When the body shunts T4 away from T3 and more toward RT3, the cells of the body slow down. This is done to conserve energy and that's normal. After the stress has passed, the metabolism is supposed to come back up to normal but sometimes it doesn't."

Doctor's Manual for WTS, pp. 20-23.

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