Re: hypothyroidism and antihistamines
The labels on cold and sinus products warn against taking them if you have "thyroid disease". They aren't specific about exactly which thyroid disease because so many people are confused about which particular thyroid disorder they have. It's a case of "dumbing down" the instructions to protect the population at large.
The warning is primarily for hyperthyroidism, not hypo. That's because the products can significantly raise pulse rate and blood pressure, two conditions that hyperT folks can already have and certainly don't need to exacerbate.
Of course, no one with high BP and pulse rate should be taking these medicines, not just hyperT patients.