Transmasculine youth (female sex assigned at birth, male gender identity) who begin clinical gender affirming hormone therapy (GAHT) with testosterone (T) may experience changes in headache. Researchers think this because studies published on effects of giving testosterone to cisgender females (female sex, female gender identity) and transmasculine adults seem to show an effect on pain. This research will help us learn more about changes in headache and in brain structure and function in transmasculine youth during the first two years of T. Youth who will be starting T within 6 months, either on puberty blocker or not, as part of their regular medical care can participate and will be asked to attend 4 visits: * before starting T * after 6 months on T * after 1 year on T * after 2 years on T At the visits, they will be asked to: * answer questions and surveys about their health * have a brain MRI done * give a small sample of blood and at their first visit, complete a physical exam. Some participants can also do a brief test of pain sensitivity. All participants will be asked to complete a headache diary for the first 6 months, for 1 month after the 1 year visit, and for 1 month after the 2 year visit.