2.3 Physiological Causes & Diagnostic Workup
Key Takeaways
- Medical mimics of psychiatric symptoms include UTI-induced delirium in older adults, hyperthyroidism mimicking anxiety/mania, hypothyroidism mimicking depression, and B12 deficiency mimicking depression, cognitive decline, or psychosis.
- Delirium has an acute onset and fluctuating course with impaired consciousness and is often reversible; dementia has an insidious onset, a progressive course, clear consciousness (early), and is usually irreversible.
- A new psychiatric presentation always requires ruling out a physiological cause before a primary psychiatric diagnosis is assigned.
- Baseline diagnostic workup typically includes CBC, CMP, TSH, B12/folate, urinalysis, and a toxicology screen; neuroimaging or EEG is added when red-flag findings are present.
- DSM-5-TR is the primary diagnostic classification system used in the United States; ICD-10-CM codes are used alongside it for billing and reporting.
TCO Knowledge statement K2 requires the PMH nurse to recognize physiological causes of psychiatric symptoms. This is a patient-safety cornerstone: assigning a primary psychiatric diagnosis to symptoms that are actually caused by an underlying medical condition delays correct treatment and can be dangerous. The rule that should guide every new presentation is "rule out medical before you rule in psychiatric."
Common Medical Mimics
Urinary tract infection (UTI) in older adults is one of the most frequently tested mimics. Unlike younger adults, older adults with a UTI often do not present with dysuria or fever — instead, they present with acute confusion, agitation, or a sudden change in behavior, which can easily be misread as a primary behavioral or cognitive disorder if a urinalysis is not obtained.
Thyroid dysfunction produces two opposite psychiatric pictures. Hyperthyroidism can mimic anxiety or mania — restlessness, insomnia, tachycardia, weight loss, tremor, and irritability. Hypothyroidism can mimic depression — fatigue, low mood, weight gain, cognitive slowing, and psychomotor retardation. A TSH is a low-cost, high-yield screening test for any new mood or anxiety presentation.
Vitamin B12 deficiency can present with depression, cognitive decline resembling dementia, irritability, or even psychosis, often accompanied by neurological findings such as peripheral neuropathy or gait disturbance. It is reversible with replacement if caught early, making it an essential screen in atypical or treatment-resistant presentations.
Substance-induced conditions (intoxication or withdrawal from alcohol, stimulants, sedatives, or other substances) can produce virtually any psychiatric symptom picture, from psychosis to mania to depression to delirium — covered in depth in Section 2.8.
Delirium versus Dementia
Distinguishing delirium from dementia is one of the highest-stakes differentiations in psychiatric-mental health nursing, because delirium is a medical emergency with a potentially reversible underlying cause, while dementia is a chronic, usually progressive condition.
| Feature | Delirium | Dementia |
|---|---|---|
| Onset | Acute (hours to days) | Insidious (months to years) |
| Course | Fluctuating, often worse at night ("sundowning") | Slowly progressive, relatively stable day to day |
| Consciousness | Impaired, reduced awareness of environment | Clear, especially in early stages |
| Attention | Markedly impaired | Relatively preserved early, declines later |
| Reversibility | Often reversible once cause is treated | Usually irreversible (progressive) |
| Common cause | Infection, medication, metabolic disturbance, withdrawal | Neurodegenerative disease (e.g., Alzheimer's), vascular injury |
A patient can also have delirium superimposed on dementia — a baseline cognitive impairment with an acute worsening from a new medical insult — which requires the nurse to compare current presentation against a documented baseline rather than assuming any confusion is simply "the dementia."
Diagnostic Workup
A structured medical workup accompanies every new psychiatric presentation, particularly first-episode psychosis, new-onset confusion, or an atypical symptom pattern. Baseline studies typically include: complete blood count (CBC) to screen for infection or anemia; comprehensive metabolic panel (CMP) to assess electrolytes, renal, and hepatic function; thyroid-stimulating hormone (TSH); vitamin B12 and folate; urinalysis; and a toxicology screen for substances. Depending on findings, the workup may expand to neuroimaging (CT or MRI, especially with focal neurological signs or first-episode psychosis in an older adult) or an EEG (when seizure activity or a specific encephalopathy is suspected).
Classification Systems
The DSM-5-TR (Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition, Text Revision) is the primary diagnostic reference used in U.S. psychiatric practice. It organizes disorders into categorical diagnostic criteria sets (the disorder families covered in Sections 2.4-2.7 of this chapter) and includes cross-cutting symptom measures that screen broadly across domains regardless of the primary diagnosis. The DSM-5 removed the older multiaxial system used in DSM-IV. Alongside DSM-5-TR, ICD-10-CM (International Classification of Diseases, 10th Revision, Clinical Modification) codes are used for billing, insurance reporting, and international epidemiological comparison; most DSM-5-TR diagnoses map to a corresponding ICD-10-CM code, and PMH-BC nurses should recognize that these two systems coexist rather than compete.
Iatrogenic and Medication-Induced Causes
Medications themselves are a frequently overlooked physiological cause of new psychiatric symptoms. Corticosteroids can precipitate mania, psychosis, or severe mood lability. Anticholinergic medications (including some over-the-counter sleep aids) can precipitate delirium, particularly in older adults. Beta-blockers and interferon are associated with depressive symptoms, and certain antibiotics and antivirals have been linked to acute confusional states. A thorough medication reconciliation — including over-the-counter drugs and supplements — is therefore part of every physiological workup, not just prescribed psychiatric medications.
Neuroimaging is not obtained routinely for every psychiatric presentation, but it becomes a priority when red flags are present: first-episode psychosis in a patient older than 45-50, focal neurological deficits, a markedly atypical presentation, or a history of head trauma. An EEG is reserved for cases where seizure activity or a specific encephalopathy (such as autoimmune or viral encephalitis presenting with rapid psychiatric change) is suspected.
The core exam-day skill from this section is pattern recognition: when a stem describes an older adult with sudden confusion, a "new" psychiatric symptom with an atypical or rapid onset, or a symptom picture accompanied by abnormal vital signs or lab values, the correct nursing action is almost always to pursue or advocate for the physiological workup before the psychiatric diagnosis is finalized.
An 82-year-old with no psychiatric history is brought to the emergency department for new-onset confusion and agitation that began this morning. Vital signs are stable and there is no fever. What should the nurse prioritize?
Which combination of features most clearly distinguishes delirium from dementia?