The assessment of thyroid function is primarily based on the symptoms, on clinical examination and on the measurement of serum TSH concentration. In hypothyroidism originating in the thyroid gland (primary hypothyroidism), the serum concentration of THS is increased, and in hyperthyroidism it is respectively decreased (in most cases not measurable). In rare hypothyroidism originating in the pituitary gland (central hypothyroidism) the serum TSH concentration is within or below the reference range and free T4 (FT4) concentration is below the reference range. Central hyperthyroidism is very rare. In this condition, TSH concentration is within or above the reference range and FT4 concentration is increased. TSH concentration is interpreted differently in patients on thyroxine replacement therapy than in diagnostics of thyroid disorders: patients on replacement therapy usually feel the best when TSH concentration is approximately 1-2 mU/l and FT4 concentration is near the upper threshold of the reference range. See also .The treatment of patients with thyroid cancer is individually tailored in specialized care. The target level of THS concentration is within or below the reference range. See also .