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Old 05-28-2008, 11:14 AM   #1
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Please Explain Half-life Of Cytomel And Synthroid

Hi,

Could someone please explain what half-life of Cytomel and Synthroid means. How long are each one of them in your system and how does half-life work?

Thanks,
Liz

 
Old 05-28-2008, 02:11 PM   #2
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Re: Please Explain Half-life Of Cytomel And Synthroid

Never mind.

Last edited by midwest1; 05-29-2008 at 07:51 AM.

 
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Old 05-29-2008, 06:06 AM   #3
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Re: Please Explain Half-life Of Cytomel And Synthroid

This is from post 21 of my thyroid care and concerns week 2 post. It goes into much more detail there on the issue, but this is the specific data you requested:

From the FDA database of pharmokinetic information:
Hormone********* Ratio in Thyroglobulin *****Biologic Potency *****t1/2 (days) *****Protein Binding (&#372
Levothyroxine (T4) *****10 - 20 *************** 1 ********************* 6-7 *************99.96
Liothyronine (T3) **********1 ******************* 4 ********************* ≤ 2 ************* 99.5

t1/2 = time to achieve 1/2 concentration or half-life. Now there is a bit of variation depending on your metabolic rate and thyroid state. If you have too much thyroid hormone in your system you will be hyperT and in an increased metabolic rate. At a hyperT merabolic rate you burn off T4 at a half-life rate of 3 to 4 days. If you are hypo your metabolism is depressed, the T4 half-life is longer, 9 to 10 days. The reasoning behind this is that your body needs every last molecule and rations it accordingly. The same holds true for T3.. but the rate is even more dramatic. Instead of a 2 day half-life you burn it up in 12-24 hours if you are hyperT or 3-4 days if you are hypoT.

Now it does take 2 weeks for your blood to equilibrate to a new dose of T4. It takes another 2-4 weeks depending on your personal metabolism and body to determine if that dosage increase is enough to suit you. It takes 7 days for T3 to equilibrate when given a steady dose and then another 2-3 weeks for your body to say YEAH or NEAH!

MG
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Old 05-29-2008, 07:12 AM   #4
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Re: Please Explain Half-life Of Cytomel And Synthroid

Quote:
Originally Posted by mkgbrook View Post
This is from post 21 of my thyroid care and concerns week 2 post. It goes into much more detail there on the issue, but this is the specific data you requested:

From the FDA database of pharmokinetic information:
Hormone********* Ratio in Thyroglobulin *****Biologic Potency *****t1/2 (days) *****Protein Binding (%)2
Levothyroxine (T4) *****10 - 20 *************** 1 ********************* 6-7 *************99.96
Liothyronine (T3) **********1 ******************* 4 ********************* ≤ 2 ************* 99.5

t1/2 = time to achieve 1/2 concentration or half-life. Now there is a bit of variation depending on your metabolic rate and thyroid state. If you have too much thyroid hormone in your system you will be hyperT and in an increased metabolic rate. At a hyperT merabolic rate you burn off T4 at a half-life rate of 3 to 4 days. If you are hypo your metabolism is depressed, the T4 half-life is longer, 9 to 10 days. The reasoning behind this is that your body needs every last molecule and rations it accordingly. The same holds true for T3.. but the rate is even more dramatic. Instead of a 2 day half-life you burn it up in 12-24 hours if you are hyperT or 3-4 days if you are hypoT.

Now it does take 2 weeks for your blood to equilibrate to a new dose of T4. It takes another 2-4 weeks depending on your personal metabolism and body to determine if that dosage increase is enough to suit you. It takes 7 days for T3 to equilibrate when given a steady dose and then another 2-3 weeks for your body to say YEAH or NEAH!

MG
MG,

Thanks for the info - it is very informative. I have been taking the Cytomel for a week today. My endo said take 1/2 of a 25 mcg tab a day to see how I do. I was so afraid of heart palpitations that I took a half of the half for the first 5 days and didn't see any difference. Still feeling like I am half dead. Anyway, yesterday I decided to take the full half (12.5) and although feeling rather jumpy I did feel some kind of a better feeling. I did take it with a little food though as I have read that you can do that. I did the same thing today. So will see how it is. I was able to get out of bed and move around for a change. When I posted my labs a while back, my endo dropped my Synthroid from .88 mcg to .75 mcg and you said that may not be enough - so we will see what my labs are in 3 weeks. I have also read about getting the T3 in a time-released form from a compounding pharmacy. That seems like it might be better. There is one of these pharmacies right near my house. So, I feel I have several options. I would almost rather feel a little hyper than dead and I mean dead!!!!!!! Guess I will have to play this out.

By the way have you heard of any experiences with the time-released T3? I read about it in several thyroid books.

Thanks for all your help,
Liz

 
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