T-shirts are one of two available shirts available in RimWorld, and are inferior to the button-down shirt. T-shirts can be purchased from traders, crafted, or stripped from Outlanders and Pirates.
As a complicated garment, t-shirts can only be made at tailoring benches, which requires complex clothing to be researched in order to be constructed. A t-shirt requires 50 of any textile, and 84 seconds of work (5,000 ticks) to be created, assuming baseline global work speed, consciousness, sight, or manipulation.
Conclusion & Comparison
T-shirts are overall not particularly useful in the game, as button-down shirts easily outplace t-shirts in terms of insulation and protection (more coverage). Although t-shirts are cheap to make, button-down shirts are barely more expensive and the only real difference is the moderate increase in labour, but this isn't significant enough to justify taking t-shirts over button-down shirts.
The following table(s) will display how good a normal quality T-Shirt is at insulating when made from various materials...