@JackAndy - I don't think with us having kids i'd want to take the risk in telling her. She knows i'm very friendly with this woman, in fact she knows she buys me gifts on my birthday and at xmas because i have told her. She just says "oh that's nice", isn't jealous in the slightest (and to be honest I think the reason i let her know is to try to get some kind of reaction from her). But if she thought I was having some kind of sexual affair with her, things could get nasty.
@PeakT - Regarding thyroid disease being treatable, well she is actually being treated for it and has been for years. She's constantly monitored too, so i'm not totally sure what's going on there. I think she likes to blame everything on her thyroid illness, for instance if she feels tired in the evening "it's my thyroid". I've tried to explain to her that things like that and not feeling 'totally refreshed' after a nights sleep happen to people with normal thyroids too. She considers us going out for the day walking around some town as exercise that requires a couple of days rest to recover from. I've learnt a couple of things during my marriage that I find helpful to stick to. I try not to waste time arguing with her over things that don't directly affect me, and more importantly I NEVER apologise if we have argued and i've upset her. If i do it's like admitting I was wrong (and she doesn't accept my apology). If i don't apologise she eventually says sorry to me. I will apologise to you now though, I realise i've just rambled on about my life story again and not kept on topic, so sorry for that!!