[Treatment and monitoring of primary hypothyroidism]

Treatment of primary hypothyroidism requires a long-term hormonal therapy with levothyroxine which restores a normal TSH value. In a series of 192 patients followed up for at least 3 years we tried to evaluate the variability of maintenance dosage and to analyse the factors predictive of replacement dosage. The L-T4 dosage varied from one individual to the other, from 0.6 to 3.2 micrograms/kg. The mean maintenance dosage (118 +/- 38 micrograms/day) was similar in men and women weight for weight, but it was higher in subjects aged under 60 and in those with Hashimoto's or congenital hypothyroidism. There is a positive linear relation between the equilibrium dose and the TSH value (n = 0.84; p < 0.001) and a negative relation with the L-T4 concentration (n = 0.71; p < 0.001). The yearly variation of dosage in each individual was small (about 3.5%) and of the same order as the variation of weight. We conclude that the L-T4 maintenance dosage depends on the cause of hypothyroidism, on the importance of biochemical abnormalities and on the patient's age, sex and weight. In practice, however, the necessary dosage is difficult to predict. In the equilibrium phase the need for replacements is remarkably stable. A yearly assay of baseline TSH seems to be sufficient to evaluate the quality of hormonal compensation in the absence of intercurrent events.