This symmetrically small thyroid gland demonstrates atrophy. This patient was hypothyroid. This is the end result of Hashimoto's thyroiditis. Initially, the thyroid is enlarged and there may be transient hyperthyroidism, followed by a euthyroid state and then hypothyroidism with eventual atrophy years later. Hashimoto's thyroiditis results from abnormal T cell activation and subsequent B cell stimulation to secrete a variety of autoantibodies.

Hypothyroidism is most often due to thyroid gland failure (primary hypothyroidism) best diagnosed with measurement of thyroid stimulating hormone (TSH), which will be high. For secondary hypothyroidism from pituitary failure, both TSH and T4 will be low. Measurement of T3 is rarely of value.