Quick question! Blood test 8am tomororw morning, should I take t3 tonight?

[janeroar]

Forgot that I should have split my t3 dose this morning and taken half of it tonight. This morning I took my usual dose of 12.5 mg T3 and 125mg T3. Should I take some T3 before bed or just try to rearrange my blood tests for another day ?

[SlowDragon Administrator]

Depends on wether results are for you or for GP/endo

Are you also testing vitamin levels

[janeroar in reply to SlowDragon]

They have been requested by my GP but want them to be useful most of all for myself. Thanks for your quick response

[SlowDragon Administrator in reply to janeroar]

I would either do test …..don’t take extra T3 tonight

Ft3 will be false low …..but if it’s for GP that’s better than too high

If test is via NHS ……very unlikely to test Ft3 …..possibly not even Ft4

Request vitamin levels are included in test

What vitamin supplements are you currently taking

If taking any supplements that contain biotin have you stopped taking 5-7 days before test

[janeroar in reply to SlowDragon]

It’s NHS and miraculously they are still testing t3.

I take magnesium, vitamin D, zinc, NAC, sometimes Selenium. Nothing with biotin in.

[jimh111]

Ideally you would have the blood taken about half way between T3 doses. However, 12.5 mcg is not a large dose. With a 24 hour half life we accumulate about 2 doses of T3. e.g. after taking 12.5 T3 we will have around 25 mcg in our blood which drops to around 12.5 mcg just before the next dose. If you deferred half your dose until 12 hours before the blood test the evening 6.25 mcg would give you roughly an extra 1.4 mcg T3 in your blood by morning. i.e. you would have about 14 mcg instead of 12.5 mcg. Add in quite a bit of of T3 being converted from T4 and you can see it makes very little difference.

I'm sure I've confused everyone including myself with the arithmetic but the point I'm trying to get across is the difference is very small. When you are on higher doses of T3 (>20 mcg?) the timing makes a big difference but for more moderate doses the difference is small.

[Aurealis in reply to jimh111]

I thought the half life for t3 was about 3 hours?

[jimh111 in reply to Aurealis]

The elimination half life of liothyronine is generally considered to be 24 hours. It is not as easy to determine as we might think. Initial studies gave radio labelled T3 and T4 (different iodine isotopes I think) to volunteers. This enabled them to separate out the T3 from liothyronine from T3 that came from conversion of T4. Another approach is to give high doses of T3, around 100 mcg and see how quickly T3 levels fall. The problem with the second approach is that the rate of T3 to T2 conversion speeds up when T3 is high.

The half life of T3 is accepted to be about 24 hours. Sometimes a 'biological' half life of T3 of around two or three days is quoted. This is misleading, it isn't a half life, it just means the effect of a dose of T3 can be felt for two days or so

[Aurealis in reply to jimh111]

thanks Jim, I seem to be out of date on this then. So Liothyronine could be a daily dose then? That is, perhaps no need to split across multiple doses?

[jimh111 in reply to Aurealis]

I think it is better to split liothyronine doses with one before bedtime.

[TiggerMe Ambassador in reply to jimh111]

Right! New year, new regime... I'd not taken it before at night as I take progesterone, but SD also recommended a late dose, so I revisited this and can see there is no interaction as far as the BNF state so shouldn't be a problem?

I don't imagine you will have needed to look into this though 😏

[jimh111 in reply to TiggerMe]

I find nighttime T3 gives me deeper and more restorative sleep.