Is thyroxine a prescription drug?
And if so, is there any other passageway to get it?
Answer: Thyroxine is a innate hormone made by your own body.
The prescription form of it is called Levo-thyroxine and it is really prescription only.
If you bring it without a medical cause to do so you will screw up your heart.
Yes, it is a prescription with the sole purpose drug. Obtaining it by any other source would not only be hazardous in you would hold no guarantee that what you were getting be the genuine article, but taking thyroxine in need medical supervision from your doctor is extremely dangerous.