Dr. Roach: Caffeine can reduce thyroxine absorption by about 30%

Dear Dr. Roach: I just read that I should avoid coffee for at least a half hour after I have taken my Synthroid, as the caffeine could reduce the medicine by up to 50%. Is this a new thing? I have been taking thyroid medication for about 40 years, and neither my doctor nor my pharmacist have ever mentioned it. It may completely rearrange my morning routine if this is true.

— D.D.

Dear D.D.: The first well-done study I found showing that coffee can reduce the absorption of thyroid hormone, like levothyroxine (Synthroid), is from 2008. This study showed about a 30% reduction in total. I have seen studies that compare taking thyroid medicine both 30 minutes and 60 minutes after having coffee. Unfortunately, tea and herbal teas also had an effect, suggesting that it isn’t just caffeine that may affect absorption.

If you do decide to change your routine, you should get retested, as your doctors have adjusted your dose to your current habits, and you may need a lower dose. If your levels are normal, and you always take your thyroxine the same way with respect to coffee, you don’t need to change your routine.

Dear Dr. Roach: I was diagnosed with small fiber neuropathy in my feet about 10 years ago. After blood tests and urinalysis, the doctors couldn’t find a cause for it, so I was prescribed gabapentin. I also take alpha-lipoic acid daily. About two years ago, I was diagnosed with celiac disease, which was a total surprise because I never had any symptoms. My neurologist at the time said that celiac disease could possibly be a cause of the neuropathy. What are your thoughts on this, and if I was to go gluten-free, how long would it take to start seeing effects, if at all?

— J.B.

Dear J.B.: Celiac disease is caused by a sensitivity to components of gluten, found in most cereal grains. While most people with celiac disease will have gastrointestinal symptoms, especially diarrhea and weight loss, others have no gastrointestinal symptoms. It is well-described that people with celiac disease may experience neuropathy.

Up to 50% of people with celiac disease may have these symptoms, especially burning, tingling or numbness in the feet. Doctors know to think of diabetes as a cause of these symptoms, but we sometimes forget that celiac disease can have these symptoms, without any gastrointestinal symptoms, or with very mild symptoms such as abdominal discomfort after eating. Patients and physicians alike may incorrectly think of these as reflux symptoms of functional stomach pain.

A gluten-free diet has been shown to improve other neurological and psychiatric symptoms of celiac disease, such as headache and depression. But, unfortunately, once small nerves are damaged, they usually do not get better despite a completely gluten-free diet. So, symptomatic treatment of the kind you are taking remains the only effective one.

I am publishing your letter because there are many people with undiagnosed celiac disease, and newer blood tests for celiac disease, such as transglutaminase-immunoglobulin-A (tTG-IgA) and endomysial-immunoglobulin-A (EMA-IgA) antibody tests, are easy to get. People with relatives who have celiac disease, those with autoimmune thyroid disease, and those with Type 1 diabetes are at a particularly high risk.

Readers may email questions to ToYourGoodHealth@med.cornell.edu.