T4 to T3 - how long ?

[Midwinter]

Hello

Does anybody know how quickly (or slowly) T4 is converted to T3? Or does it vary from one person to another?

[HarryE]

I think t4 converts to t3 based on need too. T4 is a storage hormone, so the conversion to t3, which is active in the cells, would happen when needed. For example, you need your metabolism to increase if you are cold, so if you move from a warm environment to a cold one, more t4 would convert to t3 to warm you up.

[helvella Administrator]

As a very rough and ready guide, approximately one third of the T4 a person produces gets converted to T3. (This is where some people get the "three times as potent" idea.)

So, if your thyroid produced 100 micrograms or T4 each day, then you might be converting around 33 micrograms of T4 to T3 each day. (That was chosen for simple numbers - actual amount of T4 produced could vary considerably from one person to another, and from one day to another.) You also need to factor in that the thyroid does directly produce some T3.

All sorts of things get in the way of this being the nice simple picture I have drawn above! Genes. Minerals (iron, selenium). Other hormones. Medicines. Illnesses. And, I am quite sure, many other things. So, yes, it will indeed vary between people, and within one person.

Rod

[helvella Administrator in reply to helvella]

The peak serum FT3 that occurs from taking T4 has been reported to occur about 48 hours after taking the T4 tablet. I tried to find a reference because this is from my very fallible memory, but could not - will continue to look.

Rod

[diogenes Remembering in reply to helvella]

the following may be a help;

Exp Clin Endocrinol Diabetes. 2007 Apr;115(4):261-7.

Twenty-four hour hormone profiles of TSH, Free T3 and free T4 in hypothyroid patients on combined T3/T4 therapy.

Saravanan P1, Siddique H, Simmons DJ, Greenwood R, Dayan CM.

[helvella Administrator in reply to diogenes]

Thank you, diogenes.

[jimh111]

Your question is not clear. Do you mean the T4 secreted by the thyroid or T4 taken as levothyroxine? In the case of the latter I suspect it will be a number of hours. The levothyroxine has to be absorbed by the gut and then it is deiodinased in the liver or in the cells. The cells convert T4 to T3 as they require it.

Looking at it another way when I forgot to take my liothyronine (L-T3) I began to notice an effect an hour or two after taking a tablet. As regards levothyroxine it took nearly a day before I noticed an effect and that was very small because the long half-life of T4 means the body stores quite a lot of T4.

The estimation that liothyronine is about three times as potent as levothyroxine comes from this study ncbi.nlm.nih.gov/pubmed/235... . Triiodothyronine (T3) in the blood is about four to four and a half times as potent as thyroxine (T4). This comes from studies that plotted graphs of TSH v fT3 and TSH v fT4. The difference between the tablets and the serum hormones arises because liothyronine and levothyroxine have different half-lives and absorption rates.

[shaws Administrator]

This is a link and you will see that the last three help to convert T4.

thyroiduk.org.uk/tuk/treatm...