Summary statements

• LT4 monotherapy administered once daily generally produces stable low-normal T3 levels, with seen in some thyroidectomized individuals.

• LT4 monotherapy administered once daily produces increased T4/T3 ratios, compared with native euthyroidism.

• The pharmacological equivalence of LT3 to LT4 is approximately a 1:3 ratio.

• LT4/LT3 combination therapy with LT3 given once daily produces up to 40% increase in serum T3 levels above trough levels.

• LT4/LT3 combination therapy is expected to produce 17, 44, and 88% fluctuations in serum T3 levels when 3.25, 5, and 10 ìg LT3 twice daily, respectively, is used.