Can synthroid (thyroxine) used to suppress the growth of a non-tox mng (euthyroid) continue to cause hyperthyroidism four months after discontinuing the pills?

Probably not: Your multi-nodular goiter has probably evolved from euthyroid to hyperthyroid (or toxic). Thyroid hormone from tablets could take 6- weeks or so to clear, and if the TSH was very low or undetectable, it could take longer than 6 weeks to completely re-equilibrate after stopping thyroid tablets. But if thyroid hormone levels are still high, or TSH stays low, your thyroid is probably overactive.