Is it safe to take a supplement containing 300 mg of l-tyrosine when i'm already treating my graves' disease with levothyroxine and methimazole?

Graves medications: L-tyrosine is an Amino Acid and also a precursor to thyroid hormone, l-thyroxine. It could stimulate further the hyperthyroidism of your graves. Not a good idea. Are you using the levothyroxine and Methimazole as directed by your endocrinologist? That combination is unusual, although not unheard of. One suppresses the thyroid, the other increases thyroid levels.