Serum thyroxine levels in patients receiving L-thyroxine suppression or replacement therapy.

Serum thyroxine (T(4)) and protein-bound iodine (PBI) levels were measured in 140 adult patients receiving L-thyroxine, 0.2 or 0.3 mg. daily for at least six months, as suppression or replacement therapy. All were judged to be clinically euthyroid at the time of the measurements. The mean PBI was 8.3 mug./100 ml. and the mean T(4) was 10.8 mug./100 ml. The range of values for the PBI was 3.5 to 13.2 and for the T(4) was 3.1 to 17.0. Because of the wide range of values, it is considered that the judgement of whether a patient is euthyroid on L-thyroxine therapy should be based primarily on clinical evaluation and only secondarily on serum T(4) or PBI levels.