Optimal free thyroxine levels for thyroid hormone replacement in hypothyroidism.

OBJECTIVE: To determine whether a difference exists in the free thyroxine level required to achieve a normal thyrotropin (thyroid-stimulating hormone or TSH) level between patients with primary hypothyroidism and euthyroid control subjects and compare the free thyroxine levels in patients with primary and secondary hypothyroidism receiving thyroid hormone replacement.
METHODS: We retrospectively assessed TSH and free thyroxine values in 58 patients with primary hypothyroidism and 78 euthyroid control subjects for whom screening thyroid function tests had been performed. From the medical records, we also obtained free thyroxine values for 23 patients with central hypothyroidism receiving stable levothyroxine replacement therapy.
RESULTS: The mean free thyroxine level was significantly higher in patients with primary hypothyroidism than in euthyroid control subjects (1.36 +/- 0.201 ng/dL versus 1.10 +/- 0.155 ng/dL, respectively; P<or=.0001), whereas the corresponding mean TSH concentrations did not differ significantly (1.60 +/- 1.183 mIU/L versus 1.73 +/- 0.792 mIU/L; P = .46). The mean free thyroxine value was also significantly higher in the patients with central hypothyroidism in comparison with that in the euthyroid control subjects (1.31 +/- 0.278 ng/dL versus 1.10 +/- 0.155 ng/dL, respectively; P<or=.0001), and no significant difference was noted between the patients with primary and central hypothyroidism (1.36 ng/dL versus 1.31 ng/dL; P = .60).
CONCLUSION: Patients with hypothyroidism require a higher level of serum free thyroxine to achieve a normal TSH value in comparison with euthyroid control subjects. This finding suggests that patients with central hypothyroidism should be treated to achieve free thyroxine levels in the upper part of the reference range.