Many functional medicine practitioners aim for an optimal TSH level of 0.5 to 2.5 for most adult clients. Free T4 is the more accurate test of thyroid function. The standard laboratory reference range for free T4 is 0.82-1.77 ng/dl. Free T3 (FT3): A high FT3 value indicates hyperthyroidism, while a low FT3 value indicates hypothyroidism. Optimal values ​​are values ​​above 3.2 PG/ML.