Brief Answer:
depression, ?dementia

Detailed Answer:
dear,
Welcome to HCM
As i can figure out here, the major problem is of memory loss, loss of energy, etc and a history of dental condition in past.
Her age of 67 yrs is one important thing to consider.
Secondly, you have mentioned that she talks about dying, and negative talks.

Taking into consideration the above facts i am suspecting few conditions such as depression, dementia with or without depression, etc.

Both these conditions can explain her problems of memory loss, negative thoughts and talks, loss of energy, weakness, etc.
Additionally, if there are co occurring symptoms of loss of appetite, disturbed sleep, etc it furthers supports our suspected diagnosis.
The dental condition could be an important stressor for the patient, which possibly could have triggered these events.
However before committing to any specific condition, i would like to wait for the results of MRI. Any sort of dementing changes in brain could be confirmed if we find any diffuse atrophy in brain's white matter.
There is sufficient evidence to establish a condition of clinical depression. Moreover, the underlying condition of longstanding thyroid dysfunction further adds to the risk of depression.

Let me know if she has been hypertensive or diabetic.
If there is a history of similar condition or depression in her family.

My suggestions to you are
1) wait for the results of MRI.
2) Get a vitamin B12 levels done if you have not done previously.
3) Consult a psychiatrist as she warrants a detailed clinical evaluation and history taking work up, that can evaluate her memory loss and severity of depression.

The further course of management depends upon the diagnosis. A consultation to psychiatrist is vital and highly advisable as a detailed evaluation of memory problem and clinical condition is beyond the scope of this online consultation.

Hope to have satisfied your query. Kindly revert back for more suggestions.
Wish you and your wife a good health.

Regards,
Dr Pankaj B

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