Hi, welcome to health care magic
( lighter periods) can be associated with the following conditions:(1) hormonal
problems such as throid disease- you mentioned that you have hypothyroidism
, if this is not controlled then it may impact on your periods(2) use of oral contraception
, depoprovera injections and IUD (3) inadequate diet - poor nurition can affect the periods (4) excess emotional stress (5) scaring/adhesions in the uterus
(6) as a result of menopause
The investigations that your doctor would do include: thyroid levels to assess control, other hormones that are important in reproduction such as FSH,LH. Any additional test would be done based on the history and examination.
I hope this information helps you