title: Variations in adequate levothyroxine replacement therapy in patients with different causes of hypothyroidism.
abstract: OBJECTIVE
  To compare the levothyroxine replacement dose in 181 patients with various causes of hypothyroidism.
  
  
  METHODS
  We analyzed the dose of levothyroxine used in the following five patient groups: (1) 37 patients with hypothyroidism after radioiodine therapy for Graves' thyrotoxicosis who were receiving a stable (for at least 4 years) replacement dose (mean time after 131 I therapy, 11.3 years); (2) 36 patients with Hashimoto's hypothyroidism (chronic autoimmune thyroiditis with a goiter or positive test results for antithyroid antibodies); (3) 36 patients with central hypothyroidism; (4) 36 patients with hypothyroidism after near-total thyroidectomy and 131 I therapy for thyroid carcinoma with negative total-body 131 I scans who were euthyroid when receiving levothyroxine; and (5) 36 patients with atrophic thyroiditis (no goiter and negative test results for antithyroid antibodies). Adequacy of levothyroxine replacement dose was defined as a normal thyrotropin level and clinical euthyroidism in patients with primary hypothyroidism and a serum free thyroxine index in the upper half of the normal range in conjunction with clinical euthyroidism in patients with central hypothyroidism.
  
  
  RESULTS
  The mean (+/- standard error of the mean) replacement dosage of levothyroxine (mg/kg per day) in patients with atrophic thyroiditis (1.26 +/- 0.07) was lower (P<0.05) than in patients with Hashimoto's hypothyroidism (1.59 +/- 0.07) and those with hypothyroidism after radioiodine therapy (1.56 +/- 0.05). These doses, in turn, were lower (P<0.01) than those in patients with central hypothyroidism (1.88 +/- 0.10) or euthyroid thyroid carcinoma (2.08 +/- 0.07). In a separate analysis, the levothyroxine dose in 43 patients with hypothyroidism after 131 I treatment was evaluated serially over time. The mean levothyroxine dosage increased from 0.87 +/- 0.12 at 6 months after 131 I therapy to 1.57 +/- 0.09 at 7 years (P<0.001). The serum thyrotropin concentration (in mU/mL) during levothyroxine therapy in patients with central hypothyroidism (0.31 +/- 0.08) was lower (P<0.01) than in patients with hypothyroidism after 131 I therapy (1.69 +/- 0.37), Hashimoto's hypothyroidism (1.39 +/- 0.20), atrophic thyroiditis (1.86 +/- 0.22), and euthyroid thyroid carcinoma (1.48 +/- 0.26).
  
  
  CONCLUSIONS
  The levothyroxine replacement dose varies with the cause of the hypothyroidism.
authors.affiliation: Division of Endocrinology, Allegheny General Hospital, MCP/Hahnemann University Medical School, Pittsburgh, Pennsylvania, USA.
authors.forename: M B
authors.initials: MB
authors.lastname: Gordon
authors.forename: M S
authors.initials: MS
authors.lastname: Gordon