Blood Test Guideline Question

[Gill-56]

I'm looking for some advice regarding when it's best to have blood drawn when having thyroid level tests. I'm on Levothyroxine and in the past I've fastest first and not taken medication until after the test. My Endo told me you don't need to fast and time of day doesn't matter either. Thoughts please?

[humanbean]

Your endo is wrong.

Time of day definitely affects results.

press.endocrine.org/doi/pdf...

Look at the graphs on page 2 ^^

TSH is highest in the middle of the night and lowest in the afternoon. To get the highest possible result for TSH (which is what determines dose) you need to have your test :

1) As early as possible in the day, and definitely before 9am if you can possibly manage it.

2) Don't eat or drink (except for water) before the test i.e. fast overnight.

3) The day before your test take your Levo 24 hours before the test time. So if your test is being done at 8.45am on Tuesday, take the previous Levo at 8.45am on Monday. Then on the day of the test take the Levo after the blood draw.

Number 3 is essential because Levo affects the Free T4 level in the blood. If TSH is very low or very high then Free T4 levels may be checked. The last thing you want showing up is very high levels of Free T4 - this could lead to an unwanted reduction in dose.

[Clutter]

Gill-56,

While you can be tested anytime of day and don't have to fast TSH is highest early in the morning and drops after eating and drinking so an early and fasting test can be helpful for someone with borderline high TSH to get a diagnosis or for someone with low TSH to avoid a dose reduction.

The graph in this link shows how TSH fluctuates throughout the day healthunlocked.com/thyroidu...

[SlowDragon Administrator]

Endo I saw (one of top Professors) was quite clear, that should NOT take Levo in 24 hours before test.

[Angel_of_the_North]

Your endo thinks it doesn't matter because s/he doesn't care what your TSH shows - probably rather not have to increase dose - much easier just to take 5 mins ignoring what you say and then discharge you than have to think about making a change. TSH is higher earlier in the day. The jury is still out on whether eating really lowers TSH, but best to be on the safe side.