My 9 year old daughter was just diagnosed with hypothyroidism, three years after being diagnosed with type 1. Her TSH level was 21. I wanted to ask other parents who have children with this a few questions.
At this level of TSH should she be experiencing some symptoms?
Once you started on medications how long did it take to see improvements?
She plays select Soccer and I’m torn between telling her coaches or not, any reason I should?

Everyone's different--some kids have symptoms and some don't. I'd say that it would be normal either way if she did or didn't have symptoms prior to her diagnosis. If her T3 and T4 are normal, then the TSH level has no impact on symptoms.

I'd think that improvements in symptoms should be seen pretty fast--within a couple of weeks. But it can also take some time to get the dosages under control, so that's all relative.

I don't see any reason to tell her coaches. But it should be added to whatever medical alert bracelet she wears, along with her dose of synthroid.

Concur with guitarnut, as a T1 with Hashimoto's thyroid and was a soccer coach for 15 years. Coach doesn't need to know. He is also right that getting the correct dose can take a few iterations. Also be aware as she grows, that does may change for a variety of reasons.