Don't worry its perfectly normal for them to check thyroid out - I have had a similar situation recently. Lots of late periods (since January), and my doctor has done a full bloodwork to find out why. Your thyroid gland is responsible for lots of hormones, so sometimes a thyroid disorder can cause hormonal imbalance.
However in my case my bloods were all fine. My doctor believes my hormones have been disrupted by stress and resulted in long luteal phases and delayed my periods. Apparently its really common in women who are stressed about being/not being pregnant.
I forget hun, are you trying at the moment? Because that may be causing you alot of stress and delaying your period.