Blood Test

[AppleOrchard]

I have a blood test this morning. I forgot and had a mug of tea. Will it make a difference? Should I cancel? Thank you very much.

[Lora7again]

It could make a difference so I would cancel it and re-arrange the test. You should only drink water before a blood test.

[AppleOrchard in reply to Lora7again]

I cancelled the appointment. I decided not to take the risk. I am sure all would have been okay but as I am not feeling very well I thought it would be better to reschedule anyway. Thank you for taking the time to reply.

[jimh111]

It will make no difference whatsoever (to thyroid blood tests). Studies that found a difference in TSH after food were flawed, they took blood early morning, sent the patients away for a few hours to have breakfast before taking the second blood sample. The TSH in the second sample (a few hours later) was lower! They blamed it on breakfast. When the clowns did another study a few years later (half had breakfast, half had none) they found no difference. I have seen one study where the patients had a large meal and it made a slight difference to TSH.

You are worrying about a storm in a teacup! A cup (or mug) of tea will make no difference. if it did lower TSH and hence thyroid hormones everyone would feel tired after a cuppa.

[AppleOrchard in reply to jimh111]

Thank you very much! I actually cancelled. Because the result was so different to my usual results I decided not to take the risk. I wouldn't want to wonder! I am sure it wouldn't have made a difference, but it suited me not to go because I am not feeling great anyway. I have rescheduled for next week. Thank you again for your reply.

[jimh111]

TSH rises when fT3 or fT4 falls, a feedback mechanism.

TSH also rises with TRH (Thyrotropin (TSH) Stimulating Hormone) stimulation from the hypothalamus and TSH (and fT3, fT4) will fall if TRH falls. This second mechanism is often overlooked but is very important.

It was thought that food might lower TRH and hence TSH, fT3 and fT4. Well conducted studies suggest this is not so, or possibly there is a small effect. One poor quality Indian study gave the impression food affected TSH because they took blood at different times of the day.

[jimh111]

Like the emojis! We don't actually differ:

1. Lower fT3, fT4 produces higher TSH as you point out.

2. TRH stimulates TSH (determines the 'set point'). So if TRH is reduced TSH will be lower and there will be less stimulation of the thyroid to secrete T3 and T4.

So there is a feedback mechanism (1) and a feedforward mechanism (2). Doctors often fail to appreciate this and tell patients who do not have an elevated TSH in spite of low fT3, fT4 that they are not hypo.

[jimh111]

We mustn't highjack Appleorchards' post but a normal TSH with low fT4 can be worse than a high TSH which manages to retain fT3 at normal levels. There are many causes of hypothyroidism, not just a failing thyroid gland, doctors don't appreciate this.

[AppleOrchard in reply to jimh111]

I think that’s so true. A normal TSH but low T4 is much more complicated and with a T3 mid range. My consultant always looked at the T3 in conjunction with everything else.

[Hidden]

Sorry you felt off colour and cancelled test. Thyroid test may not show diabetes. It wont be necessary to have a test for diabetes, if you have a full blood panel with vitamins B12 D and iron count. It is always wise to ask for diabetes to be included. I take my blood sugar, but thinking eight hours of fasting would be enough, was disappointed it was not and it took over 10 hours for my blood glucose to go down. If you don't have a fasting test, then if the result in England is 10 or over, then this will be an indicator that you might have diabetes as well as thyroid problems. Hope you have had advice on glutens and grains and other foods that may upset your metabolism. If the off colour feeling is due to something in the diet it may be to do with liver enzymes. Hope your test comes back normal next time.