Fasting or non fasting

[D9d9]

First blood test taken non fasting and was put on 5 mg Carbimazole. This will be my first blood test since starting medication. Should it be non fasting or fasting this time?

[jimh111]

The evidence is that fasting makes no or neglible difference to thyroid blood tests (unless fasting for days). However, most people on the forum believe it does.

Looking at your previous post you could have thyroiditis or subacute (short term) thyroiditis. This could be the thyroid playing up or a concurrent infection disrupting the thyroid, it would depend on how long you have had these symptoms.

[helvella Administrator Thyroid UK in reply to jimh111]

As I have said before, I stick to fasting for one main reason.

If we have been having our blood drawn without fasting, then we need a test that does require fasting, we will fast. That means there will be a difference which is difficult to account for. Saying the difference is small is a start, yes. But how do you compare a set of TSH, FT4 and FT3 results between two occasions when there is a factor which is known to make a difference?

As no-one appears to have done a comprehensive analysis of the difference fasting makes, including many of the commonly consumed breakfast foods, we do not know how small the difference it makes. Nor whether some foods make a much bigger difference than others.

[jimh111 in reply to helvella]

Clutching at straws! The evidence is it makes no difference except for one study that found a small difference with a 1,000 calorie fatty meal, this study hasn't been repeated. Other factors such as weather, exercise and above all time of month (if menstruating) are known to affect TSH. There's also the issue that having the blood test in the morning introduces considerable variability as TSH is changing rapidly up until midday when it levels off.

TSH is a pulsatile hormone so two tests taken under identical conditions will give different results, an average pulse amplitude is around 50%, so TSH can vary considerably even if conditions are identical. TSH is just one factor which should be reviewed in relation to fT3 and fT4 and above all signs and symptoms.

[helvella Administrator Thyroid UK in reply to jimh111]

Clutching at straws!

I look at it as avoiding one possible factor. And I did include the possibility of fasting/non-fasting also potentially having an impact on FT4 and FT3 tests.