Thank you for posting your query. I can understand your concern.
Let me solve your queries one by one.
1.
Sinus rhythm is the normal rhythm which we all have.
2. ST segment is a tracing on ECG . Nonspecific ST changes are commonly seen.They could also appear because of electrolyte imbalances, because of any inflammation of the covering layer of the heart and so and so forth. It can also be due to
anxiety or
hypothyroidism , you have.Non-specific ST-
T wave changes call for no treatment. At best, may be one can report any symptoms that an individual has to his doctor and get a repeat ECG done after six months to one year to compare the changes. But by far and large, these changes are benign and have no ill connotation.
So do not worry. Report to a clinician if you have symptoms like
chest pain, palpitations, skipped beats or breathlessness.
Hope this helps. Have a nice day.