24 yr male with 60 kg, 5.5" tall, high anti-TPO antibodies. And a slightly elevated tsh. Normal t3and t4. Not gaining weight. Shd I start thyroxine?

Hard call. Different endocrinologists will offer you different opinions. Staying trim like you are now is of course good. The question is whether your hashimoto's (which i suspect you have -- does your physician?) is slipping into hypothyroidism; only a minority do -- and if so, whether you want to take a bit of thyroxine perhaps as an experiment. Talk with yourdoctor & perhaps a 2nd opinion. Good luck.