A 45-year-old woman is referred to a PMHNP for treatment-resistant depression. She has failed two adequate SSRI trials over the past year. She reports fatigue, weight gain, cold intolerance, constipation, depressed mood, and cognitive slowing. Physical examination reveals dry skin and delayed deep tendon reflexes. No thyroid function tests have been ordered during her depression treatment. The PMHNP considers the relationship between thyroid dysfunction and psychiatric symptoms. Which of the following best describes the thyroid-psychiatric connection?