[Optimization of levothyroxine treatment. Dosage dependence on the existing parenchymal mass, age, body weight and fasting intake].

The requirement of levothyroxine (LT4) was estimated in 310 patients with various thyroid disorders. Criterion of adequate LT4 dosage was a negative TRH test after dose titration in increments of 25 micrograms/d. Dosing with concomitant food intake was tested first and, after 8-10 weeks, compared to drug intake on an empty stomach. Significant correlations to dose levels were detected. LT4 requirement was notably lower in nodular than in diffuse goitre (2.0 vs. 2.2 micrograms/kg X d), probably due to the greater mass of autonomous parenchyma in nodular goitre. This influence of parenchyma mass was demonstrated by the increased LT4 requirement in hypothyroid conditions without goitre (2.3 micrograms/kg X d) and in complete functional loss following thyroidectomy (2.9 micrograms/kg X d). LT4 demands were significantly lower in the elderly of all groups, especially in nodular goitre, obviously again due to increased autonomic hormone production. Drug intake on an empty stomach as compared to LT4 administration with concomitant food intake increased the LT4 absorption rate: switching to drug intake of similar doses on an empty stomach yielded negative TRH test results in 76% of patients and significantly reduced LT4 requirements in patients with goitre. For initiation of therapy, a dosage regimen designed according to age and body weight has proven its value. Individual adjustment of the LT4 dose by TRH testing, however, is indispensible in suppressive therapy.