Evaluate the low TSH by ordering free T4 and free T3 to clarify thyroid status, as the residual symptoms may be thyroid-related rather than inadequately treated depression.

An abnormal TSH in a patient with residual cognitive and somatic symptoms requires follow-up regardless of the psychiatric treatment status. Low TSH can indicate hyperthyroidism (fatigue can occur in both hypo- and hyperthyroidism — the "paradoxical fatigue" of hyperthyroidism is real). Free T4/T3 clarify severity. The PMHNP's role is to identify the abnormality, ensure appropriate follow-up, and avoid attributing all symptoms to the psychiatric diagnosis. Also assess for hyperthyroid symptoms (heat intolerance, weight loss, tremor, palpitations) and coordinate with the PCP or endocrinology. If free T4/T3 are normal (subclinical hyperthyroidism or normal variant), reassess other causes including SSRI sexual side effects, sleep quality, anemia, and psychosocial stress.

Increase sertraline to address the residual symptoms, since fatigue and poor concentration indicate incomplete antidepressant response requiring dose optimization.

Increasing sertraline when the mood has responded well and the residual symptoms have an identifiable medical correlate (abnormal TSH) addresses the wrong target.

Attribute the low libido to sertraline's serotonergic sexual side effects and switch to bupropion, which has a lower incidence of sexual dysfunction.

While sertraline can cause sexual dysfunction, changing medications without evaluating the thyroid abnormality may miss the actual underlying cause of the symptoms.

Recheck TSH in 6 months, as subclinical thyroid findings commonly resolve spontaneously and do not require immediate workup or intervention.

A low TSH with active symptoms (fatigue, poor concentration, low libido) requires evaluation, not passive monitoring. Symptomatic patients need timely follow-up.