Protocol for blood tests

[Caze]

Is it necessary to fast before a Thyroid function test? Another forum advises to do all blood tests fasting as early as possible so that you will be comparing like with like. Just wondered what other people do.

[SeasideSusie Remembering]

Caze

Always advised here, when having thyroid tests:

* Blood draw no later than 9am. This is because TSH is highest early morning and lowers throughout the day. If looking for a diagnosis of hypothyroidism, an increase in dose of Levo or to avoid a reduction then we need the highest possible TSH

* Nothing to eat or drink except water before the blood draw. This is because eating can lower TSH and coffee can affect TSH.

* If taking thyroid hormone replacement, last dose of Levo should be 24 hours before blood draw, if taking NDT or T3 then last dose should be 8-12 hours before blood draw. Adjust timing the day before if necessary. This avoids measuring hormone levels at their peak after ingestion of hormone replacement. Take your thyroid meds after the blood draw. Taking your dose too close to the blood draw will give false high results, leaving any longer gap will give false low results.

* If you take Biotin or a B Complex containing Biotin (B7), leave this off for 7 days before any blood test. This is because if Biotin is used in the testing procedure it can give false results (most labs use biotin).

These are patient to patient tips which we don't discuss with phlebotomists or doctors.

Following these suggestions every time means that you can accurately compare your results.

[Serendipitious in reply to SeasideSusie]

Are there any references for this information? Last week I went for a blood test through the GP for something else and without telling me the phlebotomist then tells me she’s taken blood for my thyroid too. I had taken my medication earlier that morning and also had breakfast. Plus I hadn’t stopped by B complex for a week like I normally do. I really wasn’t happy. However when I got the results back they weren’t much different to the ones I did a few weeks back with Medichecks having fasted and taken medication the day before.

[SeasideSusie Remembering in reply to Serendipitious]

Serendipitious

You could have refused the test. A medical practioner (doctor, nurse, HCA, etc) must have your informed consent to carry out any procedure. To do the procedure without your consent is against your human rights and against medical ethics. When she told you she'd taken blood for a thyroid test you just had to say "No, I don't consent to the thyroid test today, I will happily come back and do it another day but I've taken Biotin which is known to skew results, also I've taken my thyroid meds this morning and the test will be measuring my hormone levels at their peak after ingestion". They should know that about Biotin.

Scroll down to Page 13 of this pdf from the Specialist Pharmacy Service NHS England

sps.nhs.uk/wp-content/uploa...

and where it says

Primary Care Monitoring

it says:

Monitoring is by measuring TSH levels from blood tests taken prior to the morning medication.

Also Medichecks say do test before taking thyroid medication:

support.medichecks.com/hc/e...

You should not stop taking any medication that you have been prescribed by your doctor including thyroid medication. However, we recommend taking your thyroid test in the morning before you have taken your dose of thyroid medication. If you are monitoring your thyroid function over time then it is best to take your thyroid test at the same time on each occasion as thyroid hormones can fluctuate throughout the day.

and in their email confirming order is complete they say:

Do not take biotin supplements for 2 days prior to this test. If you are taking prescribed biotin you should discuss this with your doctor.

Some NHS test result reports mention that if Biotin has been taken results can be falsely affected.

A member posted this some time ago but did not include a link:

Here is information from Professor Anthony Weetman – Head of Medical Schools (UK)

Quote: The problem with using free T4 measurements if a patient is taking thyroxine is that the level fluctuate after taking thyroxine treatment. Therefore, within the few hours after ingestion there can be a ten or fifteen per cent level difference in level compared to twelve to twenty four hours after ingestion.

And this is the conclusion from another study: Therefore a fasting sample, taken in the early morning is the best method of testing thyroid hormone values. The practice of blood sample collection in the fasting usually happens early in the morning and eliminates known variations of thyroid hormone levels later in the day.

[Serendipitious in reply to SeasideSusie]

SeasideSusie,

Had I known beforehand I would have refused it. I went in for LH, FSH and Oestradiol and it was only after she did the blood draw that she told me that she’d taken blood for my thyroid. Anyway thanks for this information, I’ll save it.

Going forward, I will confirm with her what she’s testing before I sit down. I have another test tomorrow. I see your point I could have said so afterwards but I didn’t have that argument in my head. Oh well!

[jimh111]

Early as possible is suggested because TSH is higher early morning which may help if your doctor insists on a high TSH for diagnosis.

I disagree with most people who say fasting makes a difference, apart from one badly flawed study other studies show it makes no difference.

[SlowDragon Administrator]

Majority of members fast, test as early as possible in morning and make sure last dose levothyroxine is 24 hours before test

[Lalatoot]

I have my blood test as close to 2pm every time. I will have had breakfast and lunch and many cups of tea! I leave a 24 hour gap after my last dose of levothyroxine and an 8 hour gap after my last dose of liothyronine.My surgery does not offer early morning blood tests for thyroid so I do mine this way every time so I can compare like for like.

[waveylines]

I fast before the blood test so that I can have my thyroid meds after the test on an empty tummy as advised when taking this med.......I thought thats why they advised fasting ?