I have an underactive thyroid. Is it okay to get a Botox?

I would not see any problem having a cosmetic Botox treatment while having hypothyroidism.  There should be no interaction between the Botox and your thyroid medication.  

Hyper- or hypo-thyroidism should not have any effect on the results from your Botox injections.  Please consult with an experienced dermatologist to get started with Botox!