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Someone please explain what T3 does vs. T4

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Old 02-02-2005, 07:29 AM   #1
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fatiguegalore HB User
Someone please explain what T3 does vs. T4

I am learning so much about thyroid disease, but one thing I'm not clear on is the role of T3 vs. T4. I have heard that T4 is the actual working hormone, and that T3 converts to T4. Is that correct? If so, why would one need to be treated with both T4 (like synthroid) and T3 (like cytomel)?

When I took cytomel in 2003 (without any T4), it ended up pushing my T4 down below the normal range. When the endo took me off, the T4 went into normal. It is now very low normal (free T4 = .82) and TSH is now out of range (3.130).

Thanking you all for your expert guidance.

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Old 02-02-2005, 08:38 AM   #2
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dea4 HB Userdea4 HB Userdea4 HB Userdea4 HB User
Re: Someone please explain what T3 does vs. T4

T4 converts to T3. The T3 is the one that your body uses on a cellular level. It is important that your T4 stay stable in the upper 1/3 because as your body needs the T3 it is converted in the liver for use.
For some people the body does not convert the T4 to T3 well so added T3 is needed to help. This can happen for many reasons, some being... over use of alchohol, taking too much aceteminophen (tylenol) or problems with the liver or the body just plain does not, for some unknown reason, convert T4 to T3 well.
In some cases the T4 will be low and the T3 higher because the body may be trying to convert as much T4 to T3 to keep you going.

Old 02-02-2005, 10:31 AM   #3
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fatiguegalore HB User
Re: Someone please explain what T3 does vs. T4

Dea4 - Thank you for explaining this to me. I wonder why taking pure T3 would have driven down my T4? Odd. I am hopeful a pure T4 will improve my symptoms.

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