Final answer:
Ordering thyroid function tests is the correct plan for a patient showing signs of hypothyroidism and mental distress, to first rule out thyroid-related causes before making psychiatric medication changes.
Step-by-step explanation:
The correct plan for a patient with hypothyroidism and reported poor concentration and distressed state involves assessing thyroid function first. Therefore, the best next step is to order thyroid function tests. This is because the symptoms presented could be a sign that the current therapy regimen is not effectively managing the thyroid hormone levels. It is essential to routinely monitor thyroid hormone replacement in cases of hypothyroidism, which could be due to Hashimoto's thyroiditis or post-thyroidectomy treatment for hyperthyroidism, to maintain a euthyroid state and prevent symptoms of both hypo- and hyperthyroidism. While the symptoms may overlap with depression, it is important to rule out any thyroid-related causes before considering adjustments to psychiatric medications or referrals.