A 32-year-old woman complains of amenorrhea since the delivery of a baby 15 months earlier, despite the fact that she did not breastfeed. The delivery was complicated by excessive haemorrhage requiring transfusion of 2.5 litres of blood. She has also been fatigued and has gained 4.5 kg since the baby was born. Laboratory data: serum LH < 1 IU/L (normal 4-24), serum estradiol 5 pg/mL (normal 20-100), serum TSH 0.1 mU/L (normal 0.5-5), serum GH 3 ng/mL (normal < 5), serum ACTH 28 pg/mL (normal 10-50), serum prolactin 2 ng/mL (low). Injection of 500 microgram of TRH failed to produce the expected rise in both serum TSH and prolactin. Which diagnosis most likely explains these findings?