Getting a curate testing

[Shirly123]

Hi

Just had my tsh checked at 3 in the afternoon. Shocked the gp doesn't realise this is not a good time of day and no other bloodtest at the same time?

Best

Shirly

[SeasideSusie Remembering]

It doesn't shock me Shirly123 . GPs (and a lot of endos) seem to know very little about thyroid disease.

[jimh111]

Why are you shocked? Why do you believe it is not a good time?

TSH output is fairly flat between 10:00 and 20:00. It peaks at around 03:00 when it is about 0.8 mU/l higher. From a scientific point of view mid afternoon is the best time to take the blood sample as the diurnal curve is flat. If you take the blood at 09:00 you are on the downward slope of the curve and results will vary from day to day and hence be inaccurate.

If your results come back close to the upper limit of the reference interval you can point out to your doctor that if the blood was taken earlier in the day your TSH might have been 0.3 - 0.5 higher.

[greygoose in reply to jimh111]

Like he'd take any notice of that! Ha!

[Angel_of_the_North in reply to jimh111]

I thought there could be a 75% decrease over the day, and in the afternoon, mot people will have eaten so that drops TSH further.

[helvella Administrator in reply to jimh111]

If medics realised that TSH changes through the day at all, you might be able to discuss rationally. Even then, we each have our own changes in TSH level through the day. Some people hardly change at all, other change substantially. Without multiple TSH tests to demonstrate your own TSH variability, I really don't see how you can make a quantified allowance - just a general "well that was morning/afternoon so it might be a bit different".

You say that the variation is about 0.8 mU/l - the paper I have referenced several times shows one subject, number 27, changing from about 1 to 3 across the day. I think we can be fairly sure that extreme cases will very likely vary even more. (I always suspect that happens in people who became severely hypothyroid, with very high TSH. They developed the ability to make a lot more TSH than most so any factor which changes TSH might be amplified in its effect. I might be wrong.)

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

As it is, your TSH result is your TSH result without even a nod towards time of day.

Therefore, for those wanting to be diagnosed, as early as possible is sensible. And for those already diagnosed and not wanting their dose reduced due to low or suppressed TSH, as early as possible is sensible.

Also, for comparison from one test to another, get tested at the same time every time. So that means if you started being tested early, continue being tested early.

[jimh111]

From a point of view of getting an accurate result it is best to take the blood during the middle part of the day. The Russell paper helvella quotes shows that on average TSH fluctuates between 1.4 and 1.6 from 10:00 to 20:00. There is a large increase between 21:00 and 09:00. There is a large drop (from ~1.9 to ~1.55) from 09:00 to 10:00. If you have the blood taken around this time the result will be inaccurate as you will not know where on this steep slope your blood draw occurred. Even if you have each blood sample taken at exactly the same time of day your body clock will vary each day.

The strong rhythmicity subject No. 27 has a TSH of 2.6 at 09:00 and 1.4 at 10:00 (you will have to enlarge the page a lot to see the raw data dots, especially the mark at 2.6 on the axis). Thus taking the blood before 10:00 will give a meaningless result.

I've long forgotten my mathematics but the attempt to plot a sinusoidal curve using cosinor analysis seems silly. A casual look at the TSH graphs shows a fairily level TSH during the day with a large increase in the evening and night followed by a rapid drop after waking.

Some patients suggest not eating before the blood is taken. This can raise the TSH a little but if they have fasted for some time it can also substantially reduce TSH. For an accurate result it is best to have the blood taken after 10:00 and eat normally.

My real concern with attempting to nudge the TSH up a little is that it reinforces the doctors' belief that one cannot be hypothyroid unless TSH is elevated. Unless the TSH is high, or there are high antibodies it is very unlikely the patient has primary hypothyroidism (a failing thyroid gland). If they have obvious sings and symptoms of hypothyroidism with an in-interval TSH their hypothyroidism is likely to be due to other causes such as an inappropriately low TSH or some form of end organ resistance to thyroid hormone. If they are (mis)diagnosed with primary hypothyroidisim they will be (mis)treated for primary hypothyroidism and not recover, they will not receive the treatment they need which may involve much higher doses.

I know many patients have to resort to 'fiddling' the TSH assay by having the blood taken early morning in order to get any form of treatment. If you do this be aware that the results will fluctuate substantially and not a good indicator of your treatment. Where we can it is better to get an accurate TSH assay and persuade the doctor to treat based on signs and symptoms rather than TSH.