1) Thyronorm should be taken as soon as you wake up. Leaving aside water you should not have anything to eat or drink for one and a half hours after taking it.
As for
calcium tablet - yes it does effect the efficacy of Thyronorm which can be taken after lunch.
2) For the lightheadedness we need to know if you are a hypertension patient or a diabetic patient. It could also be due to cervical spondylitis. We need to rule one out after another.
Next Steps
Take a consultation for further course of action.