One of my horses has a goiter. It does fluctuate in size - sometimes it`s noticeable and at other times you wouldn`t know it`s there. For her, as she seems to be a mild case, I just make sure there`s always an iodized salt block by the water trough and there`s an iodized salt block in her feed bin as well (they get oats everyday) plus I have a mineral block. In her case, I notice she`s not a big salt block licker like the others so when she was first diagnosed with it, I put iodized table salt in with her feed for a little while until it settled down.
As the other posters mentioned, it would be a good idea to get a vet involved if they are not already. Good luck.