Hyperthyroidism causes an excess in thyroid hormone, speeding up the body's metabolism and sometimes causing weight loss as a result, even if the child's appetite per se seems OK.
Confusion often arises re: diagnosis of thyroid problems, because one of the lab tests is for TSH (thyroid stimulating hormone). Hyperthyroidism would be accompanied by low TSH levels, not high TSH levels. Be sure to pin all this down with the doctor: ask what is high (hyper....) and what is low (hypo....).
Weight gain would be an understandable result of hypothyroidism because the body's rate of metabolism slows down. TSH level would be elevated in a hypothyroid case. The levels the nurse spoke about may have been TSH level (other levels investigated in thyroid function are T3 and T4 -these would be low in the case of hypothyroidism and high in the case of hyperthyroidism).
Definitely get back to him with your questions.
That is what he said, hypo. It is why I am confused. The medication is also for hypo and gives advice about not using it for weight loss.
Are you sure the doctor didn't say hyperthyroidism rather than hypothyroidism? It is the former that is typically associated with weight loss.