Started levothyroxine 50 mcg approx 3 weeks ago. so far no change in energy level /muscle fatigue but have increased appetite and return of my menses. these two are good signs it is working, correct?

Yes correct: That little 50 mcg enter into inside the cell , to inside nucleus , the factories, the mitochondria carried inside , to stimulate every type of metabolism, dna transcription , takes little bit more time as you already showing improvements as you described , be a little patient and make sure to go for follow up to further fine tune your metabolism.

Good signs: Yes, appetite and return of menses are good signs that a benefit of medication is occurring. However, a blood draw and result review is essential to complete and monitor your progress. The tsh/thyroid stimulating hormone level goal is around 1.0. Okay to ask your doc what your level is and how long to get there. Good luck.