The alternatives are an early
Graves Disease or an overactive thyroid nodule. The ultrasound
and endocrine consultation should help get to the bottom of this.
At this age, if you have no hyper symptoms and the T4 levels are normal (would also check T3), then best to observe closely (every 6-8 weeks for the next 6 mos). The menstrual changes and infertility however may be related to the mild hyperthyroidism.