Thyroid UK

Fasting for blood tests!!

I'm finally going for my blood tests tomorrow morning and some are fasting so i just wanted to check when is the last/best time to stop drinking and eating before a fasting blood test? (obvs not Water)

I've heard 6 & 8 hours and i have also heard at 10pm the night before so wanted to check as i keep putting off getting them done and don't want to mess up the results.

5 Replies

I think I've read on here don't eat or drink for 12 hours before the test, except water as you say. If I'm wrong someone will correct me. I'm sure you already know not to take any thyroid meds for 24 hours beforehand? I always fast for at least the 12 hours when having my tests for thyroid, cholesterol and blood sugar. Good luck. :-)


Thank you...these tests aren't for my thyroid. these ones are to check my Iron, Vitamin B2, Prolactin and Testosterone levels as well as a full blood count. Last time i checked my Iron levels were within range but i didn't know you had to fast beforehand so i think they'll be back to being low but i also have Polycystic Ovaries possibly syndrome and a problem with my Lymphocytes as well as many other things to go with my overactive thyroid.

My blood forms had other things listed mainly vitamins but i had them done during my endo appointment so i'm not sure if i still need to fast. I don't currently take any meds as my thyroid is subclinical.


I hope your fasting has gone ok.

I was told 12 hours by 2 different private providers for all blood tests but only the night before by the NHS for some blood tests. For example the NHS tells you to fast sometimes for blood glucose tests but not all the time, which makes the non-fasting results useless.

Having had a discussion with a friend it seems private providers are more careful about testing even if the same labs are actually dealing with the samples.


I rang my doctors to find out and the receptionist said from 10pm the night before.

I had to go to my doctors to pick up an up to date blood test form and was told I didn't need to fast for any of them so I went straight to the hospital and had them done so I'm just waiting for the results now.

I did read online though that you did need to fast for the prolactin and as I wasn't planning on going that day I didn't so I'm worried now whether the test is gonna be accurate :(


My husband just looked this up on the internet today. Generally it is eight hours, but for cholesterol and triglycerides it's twelve, but no alcohol for twenty four hours if they are testing triglycerides.

He was most upset, as it means he can't have a beer after bowls tonight!


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