Thyroid Function Tests Reference Ranges

While cruising the Journal of Clinical Chemistry, as one does, I found a study done in Germany (2005) on the TFT reference ranges that I hadn't found before. Interesting that birth control pills can affect the mean. There are some other interesting comments as well. For those that share an interest in these things I have provided a link. PR

16 Replies

Cheers, PR4Now. That birth control pill affects TFT has been known for 20/30 years. I'll read the rest tomorrow, turning in now.

Clutter, even though I have been reading about the world of thyroid for some 25 years now I keep proving to myself that I can never know it all. I've probably read something about birth control and the thyroid, Dr. Ellen Grant might have mentioned something in her book "The Bitter Pill", but I'm finding that retaining everything I have read a real challenge. It makes perfect sense if I stop to think about it, as Diogenes explains below. If only I could add a few more terabyte of storage and re-index my hard drive to improve search function. PR

It was mentioned to me at a TFT back in the day.

PR I've only been reading thyroid for two years so I don't have so much to remember and it hasn't had to stick for so long.

I think Bart Simpson said "The more new stuff I learn, the more it pushes out the old stuff."

Clutter, "The more new stuff I learn, the more it pushes out the old stuff." Tis true, tis true. Or, the older I get, the more I forget, might also apply. PR

:-D You say tomayto, I say tomarto ;)


I think this was probably known before 2005. I was first diagnosed hyper (Graves) in the early eighties and treated with carb. It recurred periodically with varying degrees of severity. However around 1992 during yet another episode my endo concluded that because I was on combined HRT at the time it was skewing the blood results. As HRT has the same hormones as the birth control pills it can only be concluded that they must have a similar effect.

Long term it made no difference of course and I reluctantly had RAI a few years ago and then started the merry-go-round of hypo!! Oh the joy of thyroid conditions.

valjo18, "Oh the joy of thyroid conditions", I'll second that. PR

Looking at the data it shows no difference between females on OC's and not, for FT4 and FT3 but a significant change for total T4/3. This is well known from the first studies in the late 1960's and 1970's and relates to the fact that women on OC's have higher TBG levels than those not on OC's. In fact they half-mimic pregnant women in the first trimester who have even higher TBG levels. It's a direct effect of oestrogen in the pills. As the total T4 reservoirs tend to depend on TBG levels, higher TBG means higher total T4. However the FT4 corrects for such changes and thus shows no difference in the two groups.

Diogenes, I was actually searching to see if anything new had been published by Dr. Thienpont regarding the process of "harmonizing" but I did not find anything. I understand your study has been accepted for peer review, so we can be hopeful that we will be able to read it in the not too distant future. PR

Yes, we're hoping for a good outcome, having worked hard to submit as soon as we could. Essentially the study shows that no overarching reference range for TSH exists. It is affected by age, weight, size of the active gland, and most important, T4 therapy but not the sex of the subject and only marginally by smoking habit. This is also different in pregnancy, though we have not studied this condition here - a clinical trial is being planned to address this in more detail. There are however many papers showing the effects of pregnancy on TSH etc. Using an overarching TSH range means that it is set too wide, and false negatives can arise, because some people can be within the wide range but outside the narrower range specific for their situation. The relation between FT4, FT3 and TSH and the setting of reference ranges is for all parameters conditional ie based on the individual's particular physiological and genetic characteristics. Nothing is set in stone and therefore a return has to be made to carefully considering patient presentation and using tests as either confirmatory or suggesting further examination, but not as dictates in a statistical sense of the word. If and when we get the new paper published, we are going to try to depict the fluid situation pictorially but this won't start until May earliest.

Diogenes, over the last few years I have come to realize that science is a long, slow, tedious process to tease out the knowledge of our universe and its' inhabitants. Unfortunately, during this process there are also a lot of false assumptions made i.e. the TFT's are highly accurate and all you need to diagnose thyroid conditions. I look forward to reading your study when it is published. PR

I agree. Anything to do with life processes is hugely complicated. Because the genetic/physiological solutions are so widespread. That is, a given outcome can come about by many pathways. This is an inevitable result from Darwinian evolution. Multiple solutions mean optimum chance of selection of those solutions that the present environment encourages. A rigid one solution fits all wouldn't work. So we have to be variable in all solutions to our biochemical health (within limits of course). And that has escaped medical thinking.

Which all sounds like total common sense to me- that oh so elusive quality! :-/

Your paper sounds great Diogenes, thank you so much xx

I became officially hypothyroid when I came off the pill (I was on for 14 years). My TSH increased rapidly. However I've had hypo symptoms for 25 years! I reported it and there were also a few other reports that coming off the pill caused other women's hypothyroidism!

Neeta-K, interesting comment. Many of the Naturopathic and Integrative doctors that I have read feel that the pill can cause women many more problems than it solves. We are finally getting an understanding that putting synthetic hormones into a woman's body can have consequences. I sometimes wonder if the same thing will hold out for men on testosterone. It is tricky business supplementing the human body. PR

It's interesting that men and women have significantly different normal FT4 ranges. (13-21 versus 12-20). The normal ranges quoted with lab results don't seem to take gender into account.

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