Depression is most often caused by a chemical imbalance in the brain that is treated with antidepressant medicines and with psychotherapy. But medical illnesses can also cause depression and that’s why doctors do a physical exam and order some tests. We know that a clinically significant number of depression cases are attributable to medical problems but we don’t know (there is no study data) what the percent of cases are due to medical problems. Therefore, it is important to do a thorough medical and lab evaluation as outlined below if possible.
Neurological disease such as stroke, Parkinson’s disease, and multiple sclerosis can cause depression. The doctor performs a neurological exam, which if completely normal makes neurological disease as a cause of depression unlikely.
Thyroid disease can cause depression and we check for that with two blood tests: TSH (thyroid stimulating hormone) and free T4 (thyroid hormone blood level). If both are normal thyroid disease is essentially ruled out.
An elevated serum calcium level from hyperparathyroidism is a cause for depression and a normal serum calcium level (part of the comprehensive metabolic panel blood test) rules out hyperparathyroidism.
Adrenal gland over- or under-activity (Cushing’s Syndrome and Addison’s Disease) can cause depression and are screened for with the serum electrolyte blood tests (sodium, potassium, chloride, and bicarbonate—all are a part of the comprehensive metabolic panel blood test). If there is reason to suspect Cushing’s Syndrome or Addison’s Disease, then more specific tests are needed.
Liver dysfunction can cause depression and is screened for with blood tests that are also a part of the comprehensive metabolic panel (serum albumin, total bilirubin, alkaline phosphatase, AST, and ALT–all a part of the comprehensive metabolic panel).
Renal disease, as a cause for depression, is evaluated with a urinalysis and with BUN and creatinine blood tests (these two tests are also included in the complete metabolic panel).
Malignancy, infection, and connective tissue diseases (like rheumatoid arthritis or SLE) can uncommonly cause depression. They are checked for with a medical history looking for suggestive symptoms and with a physical exam and with blood tests: complete blood count (CBC), sedimentation rate (ESR), and C-reactive protein (CRP).
So, if a medical cause for depression seems possible, the doctor will want to perform a medical history (medical questions), a physical exam, a neurological exam, and order some tests.
The tests ordered might include the CBC, comprehensive metabolic panel, thyroid tests (free T4 and TSH), urinalysis, ESR, and CRP.
Two other blood tests that I recommend are a serum vitamin B12 level and a serum vitamin D level.
Usually a CT or MRI scan is not needed when looking for medical causes of depression if the neurological exam is completely normal.
Here are some videos from my youtube channel, tomwademd, about the some of the lab tests discussed in this post.
Video: Complete Blood Count
Video: Thyroid Blood Test
Video: Comprehensive Metabolic Profile (Panel)