Hyperthyroidism- follow up blood test

[Wellthisisfun]

Hi, I was started on Carbimazole (initially 20mg, increased to 40mg after speaking to consultant) nearly 5 weeks ago. I am having my first follow up blood test today. It’s at 10:50am. I know it’s best to test as early as possible but will this matter too much if I haven’t taken my morning dose and haven’t eaten yet?

I have another blood test in 2 weeks to test if it’s postpartum thyroiditis or Graves, which the consultant has sent me a special form for. I’m going to ask if they can test this today though as I’d rather know sooner than later what I'm dealing with.

thanks ☺️

[tattybogle]

Hi the instructions about early am test/ take dose afterwards / not eating breakfast before test are all for Hypothyroidism (when taking Levothyroxine) . to get the highest tSh and to avoid measuring the paek in fT4 from a levo dose that was taken a few hours before the test.

those rules don't apply for Hyperthyroidism when taking Carbimazole.

if there is a protocol for testing on carbimazole, i don't know what it is.. but i think it is usual to take medication as normal, and eat as normal, and the time of the test doesn't matter so much .

[Wellthisisfun in reply to tattybogle]

oh great, I can have some breakfast then! Thank you

[PurpleNails Administrator]

It won’t make a significant difference.

The reason it’s recommended to fast overnight. Book early morning draw & delay replacement is to show the highest TSH & lowest daily FT4.

Around 9 is the most practical time to test and see the highest daily TSH.  

Your TSH is likely to be low what ever time you test because it’s got to recover from being low & that can take time.  It can be slow to respond once low and it may remain down regulated for weeks.  Mine has stayed undetectable even though I’ve been treated for over 4 years.  

Those who take a replacement give the levels a top up every day & so it’s highest within first few hours after taking & then reduced, but carbimazole works by reducing what the thyroid can naturally make,  so the alteration is more gradual & natural.  

Stick to taking carbimazole as consistently as possible and the time of test isn’t  as important either but it is good to test under same conditions for every test.  Supplements containing Biotin can interfere with test so is applicable to any thyroid test.  

My highest TSH ever was taken very late evening & even that was under range.  

[Wellthisisfun in reply to PurpleNails]

thank you that’s very helpful and informative