Levothyroxine-half life of two weeks: I have been... - Thyroid UK

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Levothyroxine-half life of two weeks

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9 years ago•19 Replies

I have been reading & researching thyroid issues and can't understand what a "half life of two weeks means".

Can someone enlighten me please?

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19 Replies

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galathea9 years ago

A half life of two weeks means this is the amount of time. It takes for a substance to reduce by half.

For example. If you have 100 units of thyroxine.. After two weeks it will have broken down, so you have 50.. Another two weeks you would have 25. Another two weeks 12.5. Then two more weeks 6,25. And two weeks later it would be 3.125. So the time taken for 100 mg to turn into 3 mg. would be 10 weeks.

Any help?

G. X

in reply to galathea9 years ago

Yes I think so.

Is that why it takes so long to initially saturate the body after the first dose or changing doses?

Would it be fair to say in basic terms there is always;

Some just getting started but won't work properly until body is completely saturated (about 6 weeks)

Some actually working (converting T4- T3 )

Some not working and getting lost (reverse T4)

Thanks for help galathea

galathea in reply to 9 years ago

Yeah, you pretty well got it.

in reply to galathea9 years ago

Thanks

helvellaAdministratorThyroid UK9 years ago

In physics, the idea of half-life is used very successfully when looking at radio-active materials. For example, with Iodine 123 (I123) - which is used for imaging the thyroid in medicine - that has a half-life of 13 hours.

If you had, say, a gram of I123, in 13 hours you would have only half a gram. In another 13 hours you would have a quarter of a gram. Then in another 13 hours, you would have an eight of a gram. And so on. A very neat, understandable, and mathematically simple affair. (The I123 actually decays into Tellurium 123.)

In medicine, you can give someone a medicine and measure how much is in the bloodstream. For example, carbimazole has half-life of around 5.3 hours. So if you take a 20 milligram tablet, assume it gets circulated around the body almost instantly, and measure the amount in the blood, you might see a measurement of one unit or carbimazole per litre of blood (let us call that 1 U per litre). Take another sample after 5.3 hours and you would expect to see 0.5 U per litre. And in another 5.3 hours (that is, 10.6 hours after taking the tablet) you would see 0.25 U per litre.

Even then, it is extremely unlikely that the second and subsequent half-life periods see exactly the same halving of the carbimazole.

This approach works quite well for medicines which are largely unchanged in the body. They are given, they have their effect simply by being there, they are excreted. A classic of this is penicillin. In the very earliest use of it, they recovered as much as they could from the patients’ urine, to be purified and given back to the patients.

When you are dealing with thyroid hormones, they are substances which are always present in the blood. People, however low their levels, never have zero. Further, thyroid hormones are actually used up. Levothyroxine converted to T3, which is then converted to T2, etc. The rate at which they are used up also varies. You are not then able to sensibly measure every so many hours and see the amounts dropping by half at nice, neat, regular intervals of time.

So a technique is used where they make thyroid hormone (typically levothyroxine) using a radio-active isotope of iodine. Give that, see how much can be detected in the bloodstream. And keep measuring. Eventually they can see how long it takes for the amount of radio-active levothyroxine to drop to half. Do this to a healthy volunteer and it gives you some sort of estimate of how quickly thyroid hormones are passed through the body.

All these ideas of half-life are to some extent different to each. Further, we are all different. In someone who is extremely low in thyroid hormone, the first dose they are given might disappear from the bloodstream very quickly. In someone who is extremely high in thyroid hormone, any extra might circulate in the bloodstream for much longer.

When I think about T4 and T3, I always take the claimed half-lives, which vary from one source to another, with a huge pinch of salt. Then can give some very approximate idea of how fast thyroid medicines can be processed but you cannot treat them as definitive measures.

KEOB in reply to helvella7 years ago

Hi I was just looking at this idea, and wondering if you could help me. To make a long story short, I take Armour and had it retained by customs in Spain for a month, in which time I had to supplement with Spanish levothyroxine brand. Had no idea what the right equivalent dose was but I looked online and figured it out to be over 200 mcg of levothyroxine so I started to take that and felt horrible: muscle pains, slow brain, extreme fatigue. Unfortunately, no doctor here would attempt to advise me on the right dose, though they did write a report so it would get out of customs. As I felt bad, I started lowering and went down to 175 mcg. Started to feel slightly better so thought I must be doing right. Then miraculously my Armour was released.

Do I go back to taking the same exact amount of Armour I was taking for the last five yearsor are my coffers going to be full of too much T4 from the high dose of levothyroxine I took for two weeks, and should I reduce the Armour? I am afraid of going hyper (or continuing hyper if that is what I did to myself). I suppose it is possible that the body takes what it needs from the available circulating T4 and and excretes what is not used?

helvellaAdministratorThyroid UK in reply to KEOB7 years ago

I guess you were taking two grains?

If so, I'd guess that a dose closer to 150 micrograms of levothyroxine might have been closer.

You can always change back at a slightly lower dose than you were on (one and a half or one and three quarters?) and see how it goes. You do not have to commit yourself to a specific dose but can see how it goes.

You might have high T4 at present, but that will drop when you go back.

(I have never taken desiccated thyroid though I know people who have. Their opinions are likely more valuable than mine.)

I suggest you write a new post - your response down here is likely to get missed!

KEOB in reply to helvella7 years ago

Hello Helvella, I have reposted a new post. You are right 150 of levothyroxine was closer and I was taking way too much because I read one of the wrong equivalencies. I am back on Armour as of yesterday and I reduced the first day to take 90 mg, 1.5 grains, very little because I felt hyper. I think you are right the T4 is going to drop because Armour doesn't have loads of it. Still very weak though, hoping I get my strength back soon. My original dose of Armour was 2.75 grains. I will see how it goes with the 1.5 grains for a few days or maybe put it up to 2 grains and see if my body has gotten rid of the levothyroxine pretty quickly.

helvellaAdministratorThyroid UK in reply to KEOB7 years ago

Very best of luck.

Equivalences seem to have been formulated for converting safely from levothyroxine to desiccated. That is, always make the desiccated thyroid dose a bit lower to try to make sure that no-one overdoses. The effect is to make conversion the other way rather over the top!

jimh1119 years ago

The elimination half-life of levothyroxine is about one week, not two.

helvellaAdministratorThyroid UK in reply to jimh1119 years ago

Quite right - I was so busy thinking about the meaning of half-life, I read straight past the quoted time period!

in reply to helvella9 years ago

Maybe just so busy thinking about the "meaning of life"! Lol.

hbaeker in reply to jimh1113 years ago

My understanding is that the half life can vary fairly widely and is also different for those with normal thyroid function than for those with hypothyroidism.

To the best of my recollection, for those of us with thyroid issues, it is about 7 to 9 days while for those without thyroid issues it is more like 10 to 14 days.

This comes from several years ago so there may be more up to date information on it.

jimh111 in reply to hbaeker3 years ago

I think it's difficult to measure nowadays. You can either give radio-labelled thyroxine and see how long it hangs around - no ethical now. Or, you give a high dose of levothyroxine and see how quickly it goes. The problem is the half-life will change with serum hormone levels as the body tries to reach equilibrium.

Glynisrose9 years ago

Levo is a storage hormne (T4) and stays in the system for more than two weeks.

eeng9 years ago

I always wonder why doctors don't prescribe a 'loading dose' of thyroxine, i.e. a higher initial dose to get the blood levels up to something healthy, then a maintenance dose afterwards. This is what they do for someone deficient in Vitamin D (and some malaria treatments), so why not for thyroxine? Why leave people to suffer for at least a couple of weeks (as much as 6-8 weeks) while their levels improve? Having said that, I respond within a day or two to changes in my thyroxine dose, so I think the 'half life' theory is only part of the story.

in reply to eeng9 years ago

Because no matter how much Levothyroxine we initially take, the body can only absorb so much and we don't want all that unconverted T4 floating around complicating things further.

Increments should be small and every 4 - 6 weeks after testing.

hbaeker3 years ago

I've wondered that before as well.

If using a thyroid medication that also contains T3, then it can be dangerous, but if strictly T4 it seems like it ought to be possible, but there are good reasons to not do it.

I think the main reason is that there are enough factors that what might result in vastly different amounts of levothyroxine needed in different people and the doctors wouldn't want to give some amount that might be dangerous.

On the other hands, if you go off of the levothyroxine for some reason (like ran out and haven't gotten around to getting it refilled for a while -- I knew a woman who just did that), then it makes sense to start out again with the same dose as before.

I'm not going to repeat the mathematics here, but because the half life is more than a day, the amount will build up in the body and eventually reach an equilibrium of something like E=D*2^(-1/h) where E is the equilibrium in the body, D is the daily dose, and h is the half life in you in days.

helvellaAdministratorThyroid UK in reply to hbaeker3 years ago

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