Thyroid UK
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Does genetic body size/gender affect natural thyroid-related hormone levels?

I'm wondering whether body size, like gender(?) might have an impact on the amount of hormones we produce?

Are there gender-based differences in thyroid hormones? If so, are these reflected in the reference ranges given by labs? I'd ask the same regarding body size?

If I were to hypothesize that there was a relationship between body size and thyroid hormone levels and that larger people generally produce higher levels of thyroid hormones than smaller people, then I'd be inclined to believe that the smaller person who's baseline TSH levels are lower than the larger person might be presumed healthy purely because their TSH falls in the 'normal range' when infact they're hypo? Likewise the larger person whose baseline levels of TSH are higher and is hypo will be diagnosed sooner as their TSH will exceed the upper limit sooner.

This is all speculation and i'm making lots of assumptions, just something that I thought about.

I hope that if there are gender/body size/ethnicity based differences in the levels of thyroid hormones, that these are accounted for when diagnosing.

7 Replies

I think you are more likely to get a helpful response if you explain a bit more what you are thinking.



I've just updated and added more information to the question. I hope this makes things clearer.


That helps a lot in understanding! Thanks.

Some researchers/medics are also adding in age as a possible criterion.

One of the gender issues is how oestrogen affects thyroid hormones. One specific issue is that oestrogen changes the amount of thyroid binding globulin in the blood. And this has a knock on effect - but the TBG goes up and down - so every transition can have its own impact.

Some of the formulas for working out how much thyroid hormone is likely to be needed after a total thyroidectomy take into account gender and whether post-menopausal.

You can have a look at a spreadsheet I created here:



I agree with helvella. But, I would make the point that I've had the ongoing argument with my GP about my TSH etc levels being appropriate to me, rather than fitting into the ranges -- ie, she may be 'happy' with my TSH in range at 1.9, but I'm not convinced that that level is the healthiest one for me and I may well be better off with a TSH below 1.


Cinnamon, you are right, the TSH test does not reflect an individual's reference range.

Suggest you take a look at Dr. Lee's presentation at the ATA (American Thyroid Association) meeting in 2003. It is the second from the bottom called, TSH Reference Range Redefined: What Does It Mean and What Should We Do?


Sorry, that didn't take, try this.


I know of several studies on age versus FT4 and FT3. The best one studied both people who went to the doctor but were shown to be normal in thyroid function (they had something else wrong with them) and also a large panel of healthy blood donors (aged 18-63) - total number 5700 patients, 1589 donors. At most FT4 fell by about 1 pmol/L on average over a person's lifetime, but FT3 was higher in young people until 18 years, then became like an adult's level (20% less), and didn't change further in the blood donors over the 18-63 year period. With the people going to the doctor, some of them had underlying diseases that reduced their FT3 somewhat (the "low T3 syndrome" of nonthyroidal illness). Naturally this tendency increased with age - older people get sicker oftener. I don't know of any studies that relate FT4/3 levels to size (TSH can be different in obesity). And certainly there's no significant gender effect. Also in older people (70+) you occasionally get transient rises in FT4 (and TSH changes) that only last a short time and then go back to normal. Whether these are natural or drug induced, I don't know.


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