In a patient with a history of depressed mood and significant weight gain, what should be the next diagnostic step?

In a patient with a history of depressed mood and significant weight gain, the most appropriate next diagnostic step is to order a complete metabolic panel and serum thyroid-stimulating hormone (TSH) level. This approach is essential because both depression and weight gain can be influenced by underlying medical conditions, particularly thyroid dysfunction, such as hypothyroidism. Hypothyroidism can present with symptoms including fatigue, weight gain, and depression, and it's crucial to rule out these physiological causes before making a diagnosis purely based on psychiatric evaluation.

Evaluating the metabolic panel can help assess electrolyte levels, kidney function, and liver enzymes, which are important to understand the overall health of the patient and identify any potential metabolic causes for the symptoms. The TSH level specifically provides insight into thyroid function. If there is an abnormal finding, it may direct subsequent management or treatment for the patient's mood disorder.

On the other hand, while administering a Mini-Mental State Examination or having the patient complete the Patient Health Questionnaire-9 could give insights into cognitive function and severity of depression, respectively, they do not address potential underlying medical causes. Referring the patient to a psychiatrist, although could be appropriate later in the process, should not be the immediate step without first ensuring there are no medical conditions