As you say, pure speculation, but...

As others say she really is neglected by other members of the family. It's possible that she took the death of her mother very hard, but back in the days before psychiatrists and grief counselling, and lack of support from her elders, she never learnt to move on from this properly. After many years, the stress and grief becomes a matter of course. It could be a thyroid problem as you mention, it could be a chemical imbalance in the brain, or any other number of things which cause depression. It could even be many other conditions which weren't known or understood then, such as CFS or Fibromyalgia. Of course, if that's what it were, it never would have been diagnosed nor treated properly, with no drugs and no counselling, and as such her peers wouldn't take it seriously. Poor Mary!