A 43-year-old man comes to the clinic with complaints of fatigue and “forgetting things”. He says that since the birth of his fourth child three months ago, he has constantly felt “glum,” disinterested in socializing and exercising (which he previously would enjoy), irritable towards his partner and children, unable to focus on his work, increasingly indecisive, and often not hungry or able to sleep well at night. He worries about financially supporting his family. His youngest was unplanned. He has a history of moderate depression for which he has been on and off treatment over the last 10 years. He has no history of any other psychiatric illness. There is no family history of neurological disease. General, systemic, and mental status examinations reveal no abnormalities save for a slightly restricted affect. What hormonal change could be directly responsible for this patient's condition?