Timing on blood test

[Vicky789]

Hi. I have been taking t3 with t4 at about 0500 every morning. Please could someone advise me on what time would be best to have my blood drawn in order to get the most accurate and helpful results. (I am self medicating t3). Thanks

[Clutter]

Vicky789,

Ideally you will leave 12 hours between last dose and blood draw. Blood draw is best early in the morning when TSH is highest and you should fast too as TSH drops after eating and drinking. That will mean 24+ hours between last dose and blood draw so if you add 20% to your FT3 result that should give you a realistic estimate of normal circulating FT3.

[Vicky789 in reply to Clutter]

Thanks for the quick reply! Morning it is then x

[jimh111 in reply to Clutter]

Once daily drawing blood after 12 hours there will be about 71% of the T3 dose left - this will be a rough average of your 24 hour dose which falls from 100% to 50% assuming a 24 hour half-life. (the decay line is a curve hence it will fall more in the first 12 hours). If you have the blood taken 24 hours after the last dose you would need to upscale the result by 71/50 i.e. about 42%. However, I assume you would not have the blood drawn at 05:00 so if it were e.g. 10:00 you would upscale your result by 71/43 i.e. about 65%. All this assumes you are average and have average T3 half-life. Also, there's no way you are going to estimate a realistic TSH. 1. The relationship between TSH and fT3, fT4 is too complex. 2. TSH is rapidly falling during the morning, where on that slope will the blood be drawn.

As a general principle I would always have the blood drawn roughly half-way between two doses, it's simple and gives the most accurate result. I would always have the blood taken after 11:00 when TSH levels are more stable so that you can compare one result with another. I feel it's important to get accurate results so that you can realistically monitor and also perhaps because they might be useful some time in the future when thyroid hormone is better understood. I would advocate taking an assertive approach with doctors rather than trying to fiddle results.

Vicky, if I'm correct you have added a small amount of L-T3 to your dose. If it is no more than 10 mcg I wouldn't bother with the effort and expense of a blood test. If you have subsituted L-T3 for L-T4 in a 1:3 ration your previous results will be informative. If you've just added up to 10 mcg I'd just assume your fT3 is a little higher and your TSH lower. It's not worth wasting money on blood tests if there has been a small change in dose, especially if a long interval is left between dose and blood draw.

[Vicky789]

Wow jimh111, now I'm confused! I have been taking 18.5mcg t3 and 75mcg t4. Was taking 125mcg t4 before.

I will be having my blood taken at 0920. As that's when I can get it done! I have been feeling well but wanted to make sure I'm not overdoing it as no doctor supervising.

[jimh111 in reply to Vicky789]

You will be on an approximately equivalent dose. I would take about 5 or 6 mcg at 5am as this will give you a reasonable approximation to your current dose at 09:20. You can then take the remaining dose after you've had the blood taken so you won't be disrupting you regime too much.

I have found that a nighttime dose of liothyroine has more beneficial effect. Perhaps think of splitting into two daily doses.

[Vicky789 in reply to jimh111]

Thanks! I'll do that. How do you know all this stuff? It's all so complicated!

[jimh111 in reply to Vicky789]

Used to study maths but have forgotten all of it, especially after becoming hypo. I've done my best at doing the calculations but don't guarantee they are correct!!!

[G2G2]

My functional medicine doctor recommends labs be 8-10 hours after dosing with T3. If you take NDT or take T3 with levo, she suggests the same time lapse. Before labs, I take mine at night for a morning draw.

[Hidden]

My best subject was maths but it hasn't helped me with my lack of a thyroid. Nothing seems to be very logical only trial and error and lots of patience.

The help on here is great but everyone if different.