[The 509th case: fatigue, sleepiness, mood disorder, and hypothyroidism]

A 28-year-old woman presented with a 4-year history of fatigue and sleepiness and was found to have central hypothyroidism and mood disorder. The patient had normal thyroid volume and did not show any other pituitary axis involvement. Over the course of the disease, her symptom improvement matched with the free thyroxine (FT₄) rebound and the adjustment of antipsychotic medication. The patient's grandmother had central hypothyroidism, and her mother and uncle had lowered or inappropriately normal thyroid stimulating hormone. Hence, genetic involvement was highly suspected, but whole exon sequencing did not reveal a pathogenic variant. Levothyroxine tablets were prescribed to maintain a normal median level of FT₄, and mood disorder medications were adjusted by specialists. Isolated central hypothyroidism is extremely rare, and we report this case aiming to raise awareness of this condition.