[Treatment of primary hypothyroidism in adults : dosage schedule and biological surveillance (author's transl)].

Hypothyroidism in a disease with a poor prognosis, especially as the disorder is often unrecognised, or is treated empirically and without adequate supervision. Modern biological tests can establish the diagnosis, even in subclinical cases, and can also ensure close supervision of dosage to avoid long-term adverse effects of too low or too high dosage regimens. The authors prefer dl thyroxine (dl T4) to other currently employed products : wide variations in triiodothyronine (T3) activity in thyroid extracts, short abrupt action of T3, frequent intolerance to combined T4/T3. The dl T4 (or T4 at half the dosage), a prohormone active after transformation into T3 in the organism, appears to be much more convenient in use. Dosage, usually overestimated, should be an average of 165-200 microgram of 1 T4 (5 to 6 drops of dl T4). It should be progressively reduced as a function of age (approximately 16 micrograms per decade), falling to 66 microgram/day (2 drops) over 80 years of age. Dosage must be temporarily reduced by 50 p. cent or more during acute affections. When first instituting treatment it is imperative to administer the product in stages - of two weeks in the young and of one month in elderly or debilitated patients - because of the long half-life (7 days) of T4. Initial dosage should be half the predicted final dosage in the young, and even less in the elderly. If these rules are respected, the association of beta-blocking agents, in particular, becomes of less and less value. Supervision by repeated biological tests is essential, clinical investigations detecting subclinical over - or under - compensated forms with difficulty. Modern biology has completely changed the classical rules, and often permits reduction of dosage and subsequent suppression of many adverse reactions. Equilibrium is reached when TSH levels become normal and T3 levels within normal limits. This often leads to T4 levels slightly higher than normal, as the concentrations of T3, the only active compound, are derived from plasma T4 and not, as in normal subjects, from the plasma T4 (70 p. cent) and the thyroid (30 p. cent). The TRH test, to detect markedly subclinical biological forms of hypo - or hyper - thyroidism, is only rarely necessary.