Thyroid UK
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The Fast Diet

I started this last week as I've read it can help Thyroid function and that a some people (like me) who are struggling with their weight have managed to lose. It's all going fine but last night (which was a fast day) when I took my temperature it was an all time low of 33.8!!

My temp has normally been in the 34s but over the last few weeks has risen to the high 35s and even peaked at 36.1 one day, which I was really chuffed about.

I am a bit confused as to how this diet helps the thyroid when I have also read that it can hamper the conversion of T4 to T3 and am wondering if this is why my temperature was so low? Any thoughts anyone?

10 Replies

Hi, where did you read that the fast diet was good for thyroid conditions and was it from a reliable source? Everything I've read on thyroid nutrition seems to point to a consistent approach to eating/ dieting. I have had success on the fast diet before but I wouldn't do it now having been diagnosed with a thyroid condition.


You mean the 5 days eating normally & 2 days not eating anything much type thing?

I'm not a fan, but can fully understand as I have put 3 stone since my PartialT op - unfortunately I did go through a stressful time & didn't eat anything much.. not good in the long run.... actually now I lose weight on holiday eating well and doing more - I just need a permanent holiday & meds optimised! J :D

Dieting (Kent Holtorf, M.D & co)

In a highly controlled study, Brownell et al found that after repeated cycles of dieting, weight loss occurred at half the rate and weight gain occurred at three times the rate compared to controls with the same calorie intake......


I tried the 5:2 diet. Once. My adrenal problems mean I can get hypoglycaemic episodes and the fast days just didn't give me enough. Maybe that's why your temp is falling? I've been dieting for the last 2 months, and I've lost 8 lbs, but I'm doing it with healthy eating (meat, fish, lots of veg, lots of salads, fruit & nuts) and being very strict about not eating anything I don't need (no dressings, no bread, no unnecessary extras). That's what works for me, but I guess we're all different.

I don't count calories and if I start to feel hungry I eat some fruit or nuts. My usual downfall is crisps, with the occasional ice cream and biscuits thrown in. Not a single one in the house right now. I just ate a banana and crispbread with humous as a mid afternoon snack.

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There are quite a few people on the Fast Diet forum who are doing well on it and have found that their level of medication has had to be reduced, also there are posts on here from people advocating it too. I am happy on it (though its only been a week and a half!), have actually lost a little weight and it is having amazing effects on my willpower!


I found that the very low calories of fast days upset my thyroid balance. I understand now that thyroid hormones need regular food intake to work well. I have no thyroid gland so maybe that caused my dip after fasting. I would love to lose weight this way but I find a low glycemic load diet better for my overall health and weight loss.


5:2 is supposed to work better for men than for women - that's what the studies show. I know it worked for several male friends. For women, specially with thyroid problems, it might not be such a good idea. Just leaving a long time without eating (missing breakfast or dinner) *might* teach your body to burn fat and help with blood sugar problems, but it might not. But less drastic than 5:2.


The best way to lose weight is to get your T3 levels optimised. If you don't, any weight you lose will just come back and multiply.

I do not diet and do not count calories, I eat what I want, when I want. And I have lost over 30 kilos just by getting a good level of T3.

A diet where you have to fast on certain days is rarely sustainable. And once you go back to 'normal' eating, what is going to happen, do you think?

Hugs, Grey


I dare say you could level that comment at any diet really. Once you stop dieting you tend to put the weight back on. However, having read the Fast Diet book and seeing the apparent health benefits, maybe it might be sustainable on a permanent basis, who knows?


I know people who do this diet on a constant basis (not hypo.) because of the health benefits not the weight loss.

I did this and lost 24lbs (T3 only) and was very happy on it. I now do the occasional day a month and can drop 2lbs on a day. I have not gained the weight I lost but sometimes put on a couple of pounds a month if have a particularly good cake on the go - one day fasting and weight gone.

Another friend is on the fast diet and has only lost a couple of pounds but her blood pressure is back to normal having been dangerously high for years - no more statins.

Horses for courses.


Wow that's fantastic Penny. Did you struggle with your weight beforehand? I didn't even know about the health aspect before I read the book so I'm hoping I'm OK on it because I think it's a keeper!


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