As part of any settlement visa application to the UK including a fiancée visa your Thai partner will have to go for a mandatory xray for Tuberculosis and this is done with the IOM in Bangkok. Registering with them first is essential as they are recognized by the British Government, you cannot just go to any hospital.
As part of our UK Settlement visa application we help your partner by booking the appointment and taking your Thai partner for their x-ray and when it has been cleared your Thai partner will be given a certificate from the IOM certifying that your Thai partner is clear of Tuberculosis.
The certificate is valid for a period of up to 6 months so this should be taken in to consideration when submitting your visa application.
IOM website http://th.iom.int/