Hi Graeme, I can say from experience that virus's can certainly mess with the thyroid.
A few years ago when hyper, I felt shocking, tight chest, trouble breathing, no appitite so I requested my bloods checked.
My TSH came back and it had shot up from 0.02 to 0.3, it doesn't seem much, but, looking closer it did increase 12 fold.
A day or two later I came down with influenza and was quite ill for over a week. Two weeks after recovery I had routine bloods done and my TSH was back to 0.02 again.
My endo said any type of virus, infection or bug can throw the thyroid out of whack and the results won't be reliable.