DEAR DR. ROACH: I have such a minor question that I do not think you would want to publish it, but I’ve been curious for about 20 years. I am a healthy 77-year-old male without health issues. When I feel the urge to have a bowel movement, my butt sweats profusely. After the bowel movement, there’s no more sweat.

A decade ago, I asked my primary care physician to explain it. He didn’t really know. At my last annual physical, I asked the same doctor the same question. Still, no answer. I’m not really looking for a cure. I know I could apply an antiperspirant, but I’m not going to do so. I’m just looking for a physiological explanation. Got one? — B.C.

ANSWER: When you urinate, but even more when you defecate, the body switches into a nervous system mode called “parasympathetic.” (You can remember that the sympathetic nervous system is “fight or flight,” and the parasympathetic nervous system is “rest and digest.”) It is not uncommon to develop excess sweating during a bowel movement (“poop sweats”). In some people, the parasympathetic effect is enough for them to faint.

What is a bit less common is that instead of sweating all over, yours seems to be prominently, maybe only, around your buttocks. Some people have excess sweating everywhere (“generalized hyperhidrosis”), while others have excess sweating in certain locations (“focal hyperhidrosis”) such as their face or hands.

So, you have focal hyperhidrosis of the buttocks secondary to defecation. It’s a bigger-than-expected physiological reaction to a normal neurological phenomenon.

DEAR DR. ROACH: I have been taking 75 mcg of Synthroid daily for a couple years. In January 2025, my thyroid-stimulating hormone (TSH) level was 3.48 mU/L, and this past week, it was 36.5 mU/L.

I tried to think of changes that I had made, and two came to mind. I started taking fish oil, and I began to eat steel-cut oatmeal almost every day for breakfast. I have noticed that my skin is extremely dry, and I have been constipated. My mood is poor, and I can cry very easily, which is new and different.

I searched up the connection between oatmeal and thyroxine, and it appears that these two don’t go well together within four hours of each other. Have you ever heard of this? What other foods should I avoid? — M.M.

ANSWER: I am aware that caffeine-containing beverages, especially coffee, can reduce the absorption of thyroxine (Synthroid, T4, and many other brands). Milk may also reduce absorption, as can calcium supplements.

Although I have read that fiber supplements can sometimes reduce the absorption of thyroxine, the clinical relevance of this has been questioned. I have never seen a case like yours where the TSH level has increased so dramatically.

A high TSH usually means a low thyroxine (T4) or triiodothyronine (T3) level. A combination of high TSH and low T4 is most consistent with the inability of the thyroid gland to make thyroid hormone and inadequate hormone replacement. The symptoms of low thyroid you have are highly suggestive that this is the case.

While it is possible that oatmeal, a high-fiber food, is the culprit, I would want to be sure that you weren’t also drinking coffee or milk near the time of taking Synthroid. I would also check any other medications (omeprazole, antacids, cholestyramine and iron supplements are among the most common) that interfere with absorption.

Finally, I would consider other reasons for poor absorption, such as inflammatory bowel disease and celiac disease, although your constipation makes these somewhat less likely.

Dr. Roach regrets that he is unable to answer individual letters, but will incorporate them in the column whenever possible. Readers may email questions to ToYourGoodHealth@med.cornell.edu. (c) 2026 North America Syndicate Inc. All Rights Reserved