I personally think that lethargy has a very specific meaning in the medical context - would be ok with “lack of energy,” but when you say lethargic in a medical context, I am thinking of a person who is in a decreased state of consciousness. Encyclopedia Britannica refers to lethargy in the medical sense as “morbid condition of deep and lasting drowsiness from which the sufferer can be aroused only with difficulty and temporarily.”
FA23 pg 344 literally uses the word lethargy as a neuropsychiatric symptom of hypothyroidism so I think that its fine.
FA also uses hypo-activity, which I think captures the concept better. AMBOSS defines lethargy as “a state of impaired consciousness and drowsiness from which the patient can be awoken if exposed to moderate stimuli.” https://next.amboss.com/us/article/br0Hfh?q=lethargy#Z77c4f329d4e1028bea9901be93bbe0ca