Levothyroxine therapy and serum free thyroxine and free triiodothyronine concentrations
K.A. Woeber
Department of Medicine, University of California, San Francisco/Mount Zion, San Francisco, CA, USA

Although the normal thyroid gland secretes both levothyroxine (L-T4) and levotriiodothyronine (L-T3), normalization of serum TSH with LT4-replacement therapy alone in hypothyroidism is generally believed to result in a normal serum L-T3 and to reflect a euthyroid state. However several recent studies suggest that this may not be the case,. Accordingly, the relationship between serum free L-T4 and free L-T3 was examined in 20 normal individuals (group A) and in 53 patients with chronic autoimmune thyroiditis, 18 with normal TSH on no L-T4-replacement (group B), and 35 with normal TSH on L-T4-replacement therapy for hypothyroidism (group C). Data were analyzed by applying a one-way analysis of variance with correction for multiple comparisons. Serum TSH values were very similar among the 3 groups. In groups A and B, mean serum free T4 and free T3 were very similar. In group C, the mean free T4 (16±2 pmol/l) was significantly higher than the values in groups A (14±1) and B (14±2) (p<0.001) and the mean free T3 lower (4.0±0.5 pmol/l vs 4.2±0.5, NS and 4.4±0.5, p<0.02). Consequently, the mean molar ratio of free T4 to free T3 was significantly higher in group C than the ratios in groups A and B (p<0.0001), despite very similar TSH values. These findings indicate that in hypothyroid patients L-T4-replacement, that is sufficient to maintain a normal serum TSH, is accompanied by a serum free T4 that is higher than that in untreated euthyroid patients or normal individuals and may not result in an appropriately normal serum free T3 concentration. (J. Endocrinol. Invest. 25: 106-109, 2002) ©2002, Editrice Kurtis

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