The site gives some good information on Mrs. Lincoln that all lead to the idea that she might be "insane:"
Mrs. Lincoln had a history of depression dating back to childhood. Today, a doctor might diagnose her with major depressive disorder with recurrent features – moodiness and depression with accompanying hallucinations, delusions, and poor judgment. A doctor might prescribe medications or behavioral therapy.
But not in 1875.
Before her insanity trial, her hallucinations included seeing smoke spewing out of a chimney and she confided to her son Robert that Chicago was burning…long after the 1871 Chicago fire. She thought Indian spirits were taking bones from her face and pulling wires from her eyes. She stuffed her pockets and petticoat with $57,000 dollars in securities (worth more than a million dollars today), yet she thought she was impoverished.
An alternative diagnosis might have been bipolar disorder, as she had episodes of depression and periods of extreme highs and overexcitement also known as hypomania.
Her excessive shopping, during which she purchased large quantities of single items – more than 60 pairs of gloves, for example – is a symptom of her hypomania. She frequently returned all the items she bought.
But who could blame her for her behavior?
Mary Todd Lincoln had tremendous loss and sadness throughout her life. Mary’s mother died when she was 6 years old, and she was sent off to boarding school. She lost three of her four children: Eddie at age 3, Willie at age 11, and Tad at age18. Her husband was murdered while she sat by his side.