Associated with other autoimmune conditions, of course, especially diabetes , Addisons and coeliac disease.

If high TSH and low T4, goitre and positive TPO antibodies, then diagnosis clear.

Isolated high TSH is seen with some drugs and after acute illness. Persistently high TSH and normal T4 might be subclinical hypothyroidism. Treatment is recommended if symptoms/signs, especially if TPO positive as likely to become hypothyroid at some point anyway (pregnancy and infertility are other indications).

If TPO neg and no signs/symptoms, USS can be useful just to confirm evidence of thyroiditis.

Management otherwise same as congenital hypothyroidism. Maintain TSH at any point within reference range unless symptomatic (RCT of aiming for lower limit did not show any benefit, plus risk of adverse effects). 

In adults, treat hypothyroidism if two TSH results over 10 – but consider also symptoms.  For lower levels, consider 6/12 trial of treatment.