Great work, Dr. Charles

He discovers a medical cause of his patient's depression: hypercalcemia.

There exists a clever little rhyme in medicine to help students, and then doctors, remember the symptoms of high calcium levels: stones, bones, abdominal groans, and psychic moans. While it’s certainly not good poetry, it did help explain his condition.
“Stones” signify the higher prevalence of kidney stones. “Bones” refer to the leeching of calcium from the skeleton that causes osteoporosis. “Groans” describe the frequent complaint of belly pain. And finally “moans” hint at the common findings of depression, personality changes, and even temporary psychosis for those with high calcium levels.

My father struggled with hypercalcemia when he was dying of cancer. When his calcium levels were high, the difference in his mood and behavior was astonishing. When I see a patient with depression, I always wonder if there's an undiagnosed medical condition. Some other illnesses associated with depression: pancreatic cancer, heart disease, hypothyroidism, Parkinson's disease, stroke, and multiple sclerosis.

Update - An e-mailer responds:  "The very first neurologist that I saw when looking for a diagnosis told me, after running tests, that there was nothing wrong with me but stress and depression. When I asked her if stress and depression was doing 'all this' to my body should I see a psychiatrist, she said 'no, you just need to stop being a baby about being a new mother.'
Her letter to my primary reported it as 'post partum depression'.
Two years later, my drooping eye lids, double vision, shortness of breath, weak arms and legs were finally diagnosed as Myasthenia Gravis."