Increase in plasma thyrotropin levels in hypothyroid patients during treatment due to a defect in the commercial preparation

Impaired pharmacokinetics of levothyroxine in severely obese volunteers

BACKGROUND

Suppressive or replacement doses of levothyroxine (LT4) are affected by the rate and extent of the active ingredient absorbed, as well as by the lean body mass. Obesity has reached epidemic proportions worldwide and is related with many comorbidities. The aim of this study was to determine the pharmacokinetic parameters of LT4 in severely obese individuals and compared them with similar data in lean control subjects.

METHODS

We studied 62 euthyroid subjects who had negative tests for anti-thyroid peroxidise antibodies (Ab-TPO). Thirty eight of these subjects were severely obese but otherwise healthy (severe obese subjects [SOS] group). Twenty-four were healthy control subjects (control group), with a body mass index of 23.3 ± 1.7 kg/m(2). Subjects received 600 μg oral sodium LT4 after an overnight fast. Serum triiodothyronine (T3), T4, and thyroid-stimulating hormone were measured at baseline. Serum T4 and T3 was measured 0.5, 1, 1.5, 2, 2.5, 3, and 4 hours after LT4 administration.

RESULTS

Baseline serum T4 and thyroid-stimulating hormone concentrations were higher in the SOS group than in the control group; serum T3 was similar in the two groups. The corrected area under the curve and the maximum T4 concentration after LT4 administration were lower, whereas the time to maximum concentration from the baseline was higher in SOS than in the control group. The estimated plasma volume was higher in the SOS than in the control group. Mean serum T3 levels increased gradually during the four hours after LT4 administration in the control group. In contrast, they decreased gradually in the SOS group.

CONCLUSIONS

Severely obese individuals may need higher LT4 suppressive or replacement doses than normal-weight individuals due, among other factors, to impaired LT4 pharmacokinetic parameters. The latter could be attributed to their higher plasma volume and/or to delayed gastrointestinal LT4 absorption. T4 conversion to T3 might be defective in severe obesity.

[Original brands and generic preparations]

BACKGROUND

Modern guidelines of drug approval aim at interchangeability of drugs containing the same active ingredients. Therapeutic equivalence of original and generic drugs is assumed as soon as bioequivalence is documented. For this to be accepted, first, pharmaceutical equivalence must prevail (the same amount of active substances in the same dosage forms) and, second, differences in bioavailabilities must not exceed certain limits. Drastic deviations from the original--not infrequent in the past--have become rare under the new sets of rules.

CURRENT RELEVANCE

However, there is still room for sometimes stunning discrepancies between approved drugs, since the current procedures--mainly for economic reasons--do not cover some potentially relevant aspects: Usually, studies are performed on young, healthy, mostly male volunteers; possible effects of meals on bioavailability are not investigated and, after approval of a drug, maintenance of attested quality--as with all manufacturers--is not monitored. Moreover, the tolerated deviations from the bioavailability of the original drug are quite large; with certain substances, a change from the generic drug with the lowest bioavailability to that with the highest could mean transition from low efficacy to a toxic dose level. Documented examples include carbamazepine, phenytoin, levothyroxin, verapamil, and aspirin.

CONCLUSIONS

In conclusion, even today's sophisticated rules do not suffice to cover all eventualities. In particular, drugs with a narrow therapeutic range require close scrutiny in product selection. Pertinent are drug documentation as well as the distinguishing features of the respective manufacturers, mainly scientific support and record of product reliability. Besides the sometimes insufficient official documentation, the internet has been gaining importance as a source of information.