No, testosterone is not available over-the-counter in pharmacies in Thailand and requires a prescription.
Explanation
In Thailand, dangerous drugs like testosterone require a prescription or hospital administration.
A pharmacist cannot dispense dangerous drugs for off-label purposes.
Testosterone is used for hormone replacement therapy in men, and is not prescribed for building muscle.
You can buy testosterone online from a pharmacy, clinic, or doctor service that specializes in testosterone therapy. You will need to provide a doctor with your medical history, medications, and current symptoms.