How much castor oil to take internally as a tonic?

I'm using castor oil externally for many things and am having good results for most, all except the skin on my arms, which is still just as dry and scaly as ever. It was suggested that I take castor oil internally on a daily basis to help with this, and to help boost my lymphatic system. I have a goiter, and have been rubbing it into my neck at night, and my goiter is starting to shrink.

I just want to know how much to take daily, and no one seems to know. I certainly don't want it to have laxative effects every day. Is a teaspoon enough? I also take coconut oil daily, 1 tsp. 3 times a day, so this would be additional.

Thanks!