Well here's a (not very) funny thing.
I had developed a nasty rash on my cheeks and forehead, so wasn't wearing any make up while I let it calm down. On Saturday, my son stared at me and went "Ha ha - you haven't got any eyebrows!" On closer inspection, I realised that I only have the middle third of each eyebrow left, the rest has gone.
Because my brows were quite fair, and I don't wear my glasses to put my make up on, I hadn't noticed them go, but merrily pencilled along the line where I thought they should be.
A swift Google reveals that this is called the Sign of Hertoghe or Queen Anne syndrome and is a sign of hypothyroidism. Which also explains the weight loss, the mad periods, the tiredness... everything really. Sigh.
Off to see the GP tomorrow.