The primary decay mode before the most abundant stable isotope, 169Tm, is electron capture, and the primary mode after is beta emission.
The primary decay products before 169Tm are erbium isotopes, and the primary products after are ytterbium isotopes.
All isotopes of thulium are either radioactive or, in the case of 169Tm, observationally stable, meaning that 169Tm is predicted to be radioactive but no actual decay has been observed.
Thulium-170 has a half-life of 128.6 days, decaying by β− decay about 99.87% of the time and electron capture the remaining 0.13% of the time.
[1] Due to its low-energy X-ray emissions, it has been proposed for radiotherapy[6] and as a source in a radiothermal generator.