When is the best time to thyroid tests?

[Justin1967]

Is it best to take your thyroxine after the blood test or does it not matter?

[Justin1967]

The best time to draw a blood sample is either in the
morning, prior to taking your levothyroxine dosage, or
more than 9 hours after taking the levothyroxine
dosage. The reason for that is contained in a report
of one of my research studies:

Ain, K. B., F. Pucino, et al. (1993). "Thyroid hormone
levels affected by time of blood sampling in
thyroxine-treated patients." Thyroid 3(2): 81-5.

ABSTRACT: Patients receiving exogenous levothyroxine
are reported to have higher total and free serum
thyroxine levels than euthyroid controls. This may be
an artifact of the serum collection time. We explored
the effect of collection time on serum levels of
thyroid hormones in outpatients receiving
levothyroxine for replacement therapy (26 patients) or
suppression of thyrotropin (25 patients). Blood
samples, obtained during regular clinic visits (random
samples) and at more than 22 h from ingestion of
levothyroxine (trough samples), were assayed for total
and free thyroxine, triiodothyronine, and thyrotropin.
Four athyreotic patients on levothyroxine therapy had
serial blood sampling over 24 h. Compared to
corresponding trough samples, random samples had
elevated total thyroxine levels in patients receiving
replacement (8.1 +/- 1.2%, mean +/- SE, p = 0.0001)
and in patients undergoing suppression (8.8 +/- 1.6%,
p = 0.0001). Free thyroxine was increased by 12.7 +/-
2.6% (p = 0.0003) and 14.5 +/- 2.3% (p = 0.0001),
respectively, compared with trough samples.
Thyrotropin levels were 18.9 +/- 6.8% (p = 0.003)
lower in patients receiving replacement and
triiodothyronine levels showed small or no changes.
Time-course analysis showed that free and total
thyroxine levels remained significantly elevated above
baseline for 9 and 5 h, respectively, after a
levothyroxine dose. In conclusion, there is a
transient increase in thyroid hormone levels for 9 h
after an oral levothyroxine dose. Accurate assessment
of thyroid hormone levels in patients receiving
levothyroxine therapy should take this into account.
This has greatest significance in selecting minimal
levothyroxine dosages for suppression of thyrotropin.

[GravesLady]

sorry - mistakes in my first post as follew correction.

hypo later in the day because we are more hypo in the pm.

i take my meds. before test.

it doen't matter either way, unless a person has central hypothyroidism.

t4 med. last in our system up to 6 1/2 days, so it is the accumulative days,  previous days,  that show in our level and not todays.

my believe, it doesn't matter as long as every test is done at approx. the same time.

[Applecore]

Do you take your meds before the test.  I always notify my endo if I take them or don't take them.  I never know which is better but I figure to be consistent in either is better.   I am hypo and I have always tested in the early morning with no meds.  Is that good?  I didn't know there was a better time.  Boy I have a lot to learn.  thanks for all your help and knowledge.   linda

[GravesLady]

From what I have read it doesn't matter what time of day.  However, I also have read that hypos be tested as late in the afternoon as possible because we are more hyper later in the day.  Likewise, hyper test as early in the AM as possible because we are more hyper in the AM.

I am treated Hyper/Graves' and I test in the AM when Lab opens.