What is the most appropriate initial investigation for a patient presenting with depression, loss of appetite, social isolation, and substance use, including marijuana?

Initial Investigation: Thyroid Function Test

The most appropriate initial investigation is D. Thyroid function test (TFT), as thyroid dysfunction must be excluded before diagnosing primary psychiatric disorders, since it can fully explain all of this patient's presenting symptoms including depression, anxiety, appetite changes, social withdrawal, and medication non-compliance. 1

Rationale for Thyroid Screening First

• The American Psychiatric Association emphasizes that medical causes of depressive symptoms must be treated before diagnosing primary psychiatric disorders, and thyroid dysfunction is a readily treatable condition that can present with this exact constellation of symptoms 1
• Thyroid dysfunction is a common medical mimic of psychiatric illness, and the American College of Psychiatry recommends avoiding initiation of psychiatric treatment before excluding this reversible cause 1
• This patient's presentation—depression, loss of appetite, social isolation, anxiety, and medication non-compliance—are all classic manifestations of thyroid disease that could be completely resolved with appropriate thyroid treatment 1

Why Other Tests Are Secondary

Urine drug test (Option B) should be performed as part of the comprehensive psychiatric evaluation, but it is not the initial investigation because:

• While the American Psychiatric Association recommends assessing substance use history including marijuana as part of the initial psychiatric evaluation 1, and urine drug testing is appropriate for patients with suspected substance use 2, this test only confirms what the patient has already disclosed (marijuana use for stress relief)
• A positive urine drug test does not change immediate management and does not rule out treatable medical causes 2
• The patient is openly discussing marijuana use, making covert testing less clinically urgent than excluding thyroid disease 2

Complete blood count (Option A) may be indicated as part of a comprehensive metabolic workup but:

• CBC is recommended for eating disorder evaluations 2 and general medical screening, but does not specifically address the most likely reversible medical cause in this presentation
• Thyroid dysfunction takes priority as it directly explains the psychiatric symptomatology 1

Pulmonary function test (Option C) is not indicated because:

• While cannabis use may affect respiratory function 2, this patient presents with psychiatric symptoms, not respiratory complaints
• Pulmonary testing does not address the presenting psychiatric symptoms and would not change acute management 2

Clinical Approach Algorithm

1. Order thyroid function tests immediately to exclude hypothyroidism or hyperthyroidism as the primary cause 1

2. Simultaneously obtain urine drug screen to document substance use patterns, as this informs both diagnosis and treatment planning, particularly given the patient's stated intention to use marijuana and medication non-compliance 1

3. Screen for suicidal ideation using validated tools such as PHQ-9 for depression and GAD-7 for anxiety, as depression with substance use carries elevated suicide risk 1

4. Assess cannabis use patterns specifically: daily consumption amount (>1.5g/day smoked or >20mg/day THC oil indicates high risk for withdrawal and psychiatric complications), duration of use, and whether use preceded or followed psychiatric symptoms 2, 3

Critical Pitfalls to Avoid

• Never initiate antidepressant or anxiolytic therapy before thyroid results return, as treating psychiatric symptoms of undiagnosed thyroid disease will fail and delay appropriate treatment 1
• Do not prescribe benzodiazepines for this patient's anxiety given the substance use history and stated intention to use marijuana, as this poses extremely high relapse risk 1
• Recognize that marijuana use worsens depression and anxiety symptoms (B=1.24 for depression, B=0.80 for anxiety, both p<0.05) and leads to poorer mental health functioning, so counseling about cessation is essential regardless of thyroid results 4
• Cannabis may exacerbate underlying psychiatric conditions rather than relieve them, and depression often drives increased cannabis use frequency rather than the reverse 2, 5

Documentation of Substance Use

• The American Psychiatric Association recommends comprehensive assessment of tobacco, alcohol, marijuana, and misuse of prescribed medications as part of the initial evaluation 1
• Document specific cannabis consumption patterns: frequency (daily vs. occasional), amount, route (smoked, oil, edible), THC vs. CBD content, and temporal relationship to psychiatric symptom onset 3
• Cannabis use disorder occurs in 10% of chronic users and is associated with clinically significant impairment including using more than expected and difficulty cutting back 2