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time of day to have blood work on Synthroid??

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Old 06-11-2003, 10:58 AM   #1
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michele32871 HB User
Question time of day to have blood work on Synthroid??

Hello all, hope someone can help. Does anyone know if TSH levels peak at a certain time after taking Synthroid? I thought I read here that you should not have your blood drawn within a certain amount of hours of taking your synthroid , because your levels peak and your test may not be accurate. Maybe I read that your TSH peaks around 4 hours after taking Synthroid? Not really sure, have to have levels retested, any advice??? thanks
Michele from PA

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Old 06-11-2003, 07:56 PM   #2
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SvenskaFlicka HB User

It has been many years since I read anything regareding this so I might be a little off, but here it is.

TSH releases in a pulsatile manner every 1 hr 40 min and peaking at approx 2AM-3AM with the lowest in the late afternoon. Best for hypos to get tested in late afternoon as possible. Hypers to have test done early as possible in the mornings.

As far as blood drawn within a certain amount of hours of taking synthroid, your test will reflect what is already in your system which has been stored for several days, and not that days dosage. I believe it takers 6 days for full effects in your system.


Old 06-12-2003, 10:27 AM   #3
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Meep HB UserMeep HB UserMeep HB User

For most people, Flicka is right on the money. TSH usually peaks at night after you go to bed. This isn't true for all people, but as a general rule, it is correct.

For T4 levels (and T3 is a similar situation, for people who are taking a med with T3 in it), here is what the online thyroid textbook at [url=""][/url] says on peak blood levels after taking your meds:

Serum T4 concentrations peak 2 to 4 hours after an oral dose and remain above normal for approximately 6 hours in patients receiving daily replacement therapy. The long half-life of thyroxine of about 7 days allows treatment with a singly daily tablet...
and for you T3 takers:
Liothyronine. After oral administration of L-triiodothyronine sodium (which is more readily absorbed than T4) peak levels of serum T3 are observed within 2 to 4 hours...The half-life of T3 is approximately one day.
In plain English, after you take your pill, your blood level of T3 and/or T4 is artificially high , peaking 2-4 hours after you take the pill and remaining above your base level for 6 hours.

My opinion is that you shouldn't have your blood drawn for about 8 hours after you take your meds if you are having T3 and T4 measured.

If the blood draw is in the morning, wait to take your meds until AFTER the blood draw.

If the blood draw is in the afternoon, schedule it as late as possible and take your meds as early as possible in the morning.

If all you are having done is TSH, my understanding is that TSH is going to be based on your base level, anyways and shouldn't matter on timing in respect to when you took your meds, though it will vary depending on your circadian rhythm.

I am not a doctor, nor have I ever played one on TV...

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