The FastDay Forum

[Betsysgr8]

I was reading a post by @carorees posted in another thread and it got me to wondering. How long after breaking fast until you should have the blood tests done? I'm schedule for (TSH), (T4, free, direct) and (T3, free) in next couple of weeks and want to know when is the best time to get a "true" picture of my thyroid levels? Last time I was tested, my TSH was at or near 0, giving her the indication that I'm a bit hyper but I know my body and I know when I'm hyper and hypo and I'm NOT the least bit hyper thyroid at the moment. So, this post made me think that fasting had skewed the test results a bit.

Yes, the studies I've read on thyroid function during fasting were done in normal people. The changes are quite complex! This study looked at the dynamics of thyroid function during fasting: http://www.ncbi.nlm.nih.gov/pubmed/6403568

This study in fasting, mildly obese females suggests that: 1) The transient suppression of serum TSH during early fasting is not TRH mediated. 2) Fasting does not alter plasma TRH levels. 3) A temporal sequence of changes in serum thyroid hormone indices occurs in fasting, this being an initial rise in FT4 (10 h) followed by a fall in both serum TT3/TT4 (12-14 h) and TSH (30-36 h) and finally by a rise in TrT3/TT4 levels (48 h). This sequence of events suggests that the initial inhibition of serum TSH levels in early fasting results from the acute elevation in FT4 levels, and that the reestablishment of normal serum TSH levels with continued fasting is associated with declining serum TT3 levels.

Here are some more papers showing the effect of fasting on thyroid function:
http://www.ncbi.nlm.nih.gov/pubmed/6695550 looked at the effects of a 30-hour fast followed by an 800 cal meal in healthy men and showed changes in thyroid function.
http://www.ncbi.nlm.nih.gov/pubmed/12727933 looked at fasting in men
http://www.ncbi.nlm.nih.gov/pubmed/16714386 looked at fasting in women

I went through a phase of thinking I was Hypothyroid, my bloods were all over the place, but never quite went over the magical figure doctors stick to like glue in the UK. So I joined thyroid forums and read a lot.
What all the sufferers were say was that you book your test as early as possible, you DON'T take your meds until after the test as that will give a true reading of what your thyroid is doing, not what the tablets are doing.

[Betsysgr8]

I went through a phase of thinking I was Hypothyroid, my bloods were all over the place, but never quite went over the magical figure doctors stick to like glue in the UK. So I joined thyroid forums and read a lot.
What all the sufferers were say was that you book your test as early as possible, you DON'T take your meds until after the test as that will give a true reading of what your thyroid is doing, not what the tablets are doing.

Does this hold true if your thyroid is virtually non-functioning through radioactive-iodine treatment?

I was diagnosed with Graves disease 20 years ago and had the treatment (18 yrs ago) because I developed a reaction to the thyroid suppression treatments and with my HR over 120bpm resting couldn't NOT do the treatment. When for some reason I missed a dose of levothyroxine I feel "hypo" very quickly sometimes the same day but definitely by next morning.

The idea is to take your medication with you and take it as soon as the blood is taken, in the nurses office if you can. If not as you walk to the door with a swig of water from a bottle. That way you are only an hour late with your meds, not a day late.

A quick read of those articles makes me think that the best time to test would be after a feed a day, or maybe even after 2 consecutive feed days.

I always have my blood work done first thing in the morning, after an overnight fast of about 10hrs, before I take my levothyroxine for the day. When you take your pill it causes a surge in T4 for about 6hrs which should result in a depression in TSH, hence better to not test within 6hrs after taking it. I think the MOST important thing is to be consistent.