What is the most appropriate initial investigation for a patient presenting with depression, loss of appetite, social isolation, and a desire to use marijuana for stress relief, who also has a history of non-compliance with anxiety and depression medications?

Initial Investigation: Thyroid Function Test

The most appropriate initial investigation is D. Thyroid function test (TFT), as thyroid dysfunction must be excluded first before confirming a primary psychiatric diagnosis, since it can fully explain all presenting symptoms including depression, loss of appetite, social isolation, and medication non-compliance. 1, 2, 3

Rationale for Prioritizing Thyroid Screening

Medical causes of psychiatric symptoms must be ruled out before initiating psychiatric treatment. 1, 3 Thyroid dysfunction is a common medical mimic that can present with:

• Depression and anxiety symptoms
• Appetite changes (loss of appetite in hyperthyroidism, increased in hypothyroidism)
• Social withdrawal and isolation
• Cognitive dysfunction affecting medication compliance
• Fatigue and loss of motivation 2, 3

Thyroid dysfunction is readily treatable and can completely resolve psychiatric symptoms when corrected. 3 Starting psychiatric medications without excluding thyroid disease risks missing a curable medical condition and exposing the patient to unnecessary psychotropic side effects.

Why Other Options Are Less Appropriate

Complete Blood Count (CBC)

• CBC is recommended for eating disorder evaluations specifically, not for general depression/anxiety presentations 3
• Does not address the immediate need to exclude medical causes of psychiatric symptoms
• Would not change initial management in this clinical scenario

Urine Drug Test

• While substance use assessment is important, urine drug testing does not change immediate management and does not rule out treatable medical causes 3
• The patient has already disclosed intent to use marijuana—confirmation via urine test adds no diagnostic value
• Cannabis use assessment should focus on consumption patterns (frequency, amount, temporal relationship to symptoms) rather than binary detection 3
• Testing may damage therapeutic alliance when the patient is being transparent about substance use intentions

Pulmonary Function Test

• Not indicated for psychiatric presentation
• Only relevant if respiratory symptoms were present or if chronic cannabis use had caused pulmonary complications
• Does not address the core clinical question

Comprehensive Assessment After TFT

Once thyroid dysfunction is excluded, proceed with structured psychiatric evaluation:

Validated Screening Tools

• Use PHQ-9 for depression screening (scores: 0-4 minimal, 5-9 mild, 10-14 moderate, 15-19 moderately severe, 20-27 severe) 1, 2, 4
• Use GAD-7 for anxiety screening (scores: 0-4 minimal, 5-9 mild, 10-14 moderate, 15-21 severe) 1, 2, 4
• Immediately assess PHQ-9 item 9 for suicidal ideation regardless of total score—any endorsement requires emergency psychiatric evaluation 1, 2, 4

Medication Non-Compliance Exploration

Assess specific barriers to medication adherence: 2

• Fear of side effects or addiction
• Cost concerns
• Embarrassment or denial of illness
• Frustration with polypharmacy
• Lack of perceived benefit

Common pitfall: 50-60% of patients with depressive disorders have comorbid anxiety, and standard practice is to treat depression first when both are present. 1, 2 This patient's non-compliance may reflect inadequately treated comorbid conditions.

Cannabis Use Pattern Documentation

Document specific details: 3

• Daily consumption amount and frequency
• Duration of use
• Route of administration (smoking, vaping, edibles)
• THC versus CBD content
• Critical: Did cannabis use precede or follow psychiatric symptom onset? 3

Evidence shows depression often drives increased cannabis use frequency rather than cannabis relieving symptoms. 5 Cannabis may exacerbate underlying psychiatric conditions. 3, 6

Functional Impairment Assessment

Document specific impairments in: 2

• Work performance
• Social relationships and isolation behaviors
• Activities of daily living
• Concentration and memory

Management Based on Severity

For moderate symptoms (PHQ-9: 10-14, GAD-7: 10-14): Consider low-intensity interventions including cognitive behavioral therapy and consultation with mental health professionals for diagnostic confirmation. 2, 4

For moderate-to-severe/severe symptoms (PHQ-9: 15-27, GAD-7: 15-21): Immediate referral to psychiatry/psychology for formal diagnosis and high-intensity treatment is mandatory, as symptoms at this level markedly interfere with functioning. 1, 2, 4

Critical Pitfalls to Avoid

• Never dismiss the patient's marijuana use intent without proper assessment—this increases anxiety and reduces trust 2
• Do not initiate psychiatric medications before excluding thyroid dysfunction 3
• Avoid benzodiazepines in patients with substance use history due to high relapse risk 3
• Monitor medication compliance biweekly or monthly until symptoms remit, as patients with depressive symptoms commonly lack motivation to follow through on treatment 1