T3 / T4 what is the differernce and which is levothyroxine?

[tinkeraves]

Hi. I have been diagnosed wit hupothyroidism for over 11 years. Currently I am taking 125mg of levothyroxine a day. Can someone tell me the difference in T3 and T4? Thank you

[jimh111]

Levothyroxine is L-T4, L standing for 'levo'. The thyroid also produces a little dextrothyroxine (D-T4) which has limited action, so they just make the levo form. T4 is converted by the body to T3 by removing an iodine atom, T3 is the active hormone. Levothyroxine is preferred because it has a seven day half-life which gives stable levels with a daily tablet. T3 has a 24 hour half-life, the tablets Liothyronine (L-T3) thus give fluctuating levels even if you split it into two daily doses. The body converts T4 to T3 as and when it needs it, BUT levo does not work for all patients. So, if you do well on levothyroxine fine, continue with it, but many patients need varying amounts of liothyronine. I assume that because you came on this forum you are not doing well, in which case it would be useful if you have some blood test results and a description of symptoms.

[tinkeraves in reply to jimh111]

Thank you. That explains it better.