In this context, euthyroidism refers to the amount of hormone present.
What is a goiter:
The term “goiter” simply refers to the abnormal enlargement of the thyroid gland. It is important to know that the presence of a goiter does not necessarily mean that the thyroid gland is malfunctioning. A goiter can occur in a gland that is producing too much hormone (hyperthyroidism), too little hormone (hypothyroidism), or the correct amount of hormone (euthyroidism). A goiter indicates there is a condition present which is causing the thyroid to grow abnormally.
See also here and here for more of the same info.
ETA: According to this:
Chronic lymphocytic thyroiditis (CLT), also known as Hashimoto thyroiditis, is an autoimmune, inflammatory process that causes up to 55% to 65% of all euthyroid goitres...
Although, as the link I already gave above state, regular ol' iodine deficiency can still cause euthyroid goiter.