I know i have a problem with my thyroid, but i have tried synthroid, (thyroxine) but i think it has to do with potassium deficiency, is that possible for the weight gain, fluid retention, charlie horses,?

Not likely: You have to stop guessing.If you have a thyroid problem and if you were prescribed Synthroid (thyroxine) means you have Hypothyroidism,you have to continue and follow up with your Doctor who prescribed the Synthroid (thyroxine) as you will need repeat blood test to see if you need any dose adjustment. Pottasium deficiency does not cause weight gain and fluid retention,ask your doctor to check potassium level as well