Iodine deficiency can cause goiter but they aren't likely to have an iodine problem if they eat any pellets or people foods. If they get only seed/veggies, then it's possible.
Back when my first budgie developed a growth on his neck, my vet did ask about iodine in his diet - at the time, I was giving him a tiny bit of kelp powder for iodine, and she said that was good and unlikely to cause an overdose of iodine since it was natural (the growth was not a goiter, though, so I can't say if the kelp powder was needed or helpful - it just seemed harmless).