Effect of Age, Weight and Sex on Levothyroxine Pharmacokinetics

Hypothyroidism Treatment in Aging and Thyroid Cancer

This study was a prospective, single-center, open-label, non-randomized, pharmacokinetic study using stable isotope carbon-13 labeled levothyroxine

Setting The study was conducted on the clinical research unit of an academic medical center.

Participants Adults of any age being treated with levothyroxine for hypothyroidism were studied.

Interventions A single dose of 13C- LT4 was administered to hypothyroid subjects taking levothyroxine replacement.

Main Outcomes and Measures Eighteen serial plasma samples were collected. One sample was obtained before the 13C- LT4 dose and the remainder over the 312-hour period post-dosing. 13C- LT4 concentration was quantified using validated liquid chromatography tandem mass spectrometry methods. Pharmacokinetic analysis was conducted using linear log trapezoidal non-compartmental analysis using Phoenix 6.4.

Inclusion Criteria:

• age >21 years at the time of consent
• euthyroidism while undergoing treatment with LT4
• no other serious illness
• ability to give written informed consent.

Exclusion Criteria:

• baseline hematocrit lower than 28.0%
• TSH greater than 4.5 mIU/L
• kidney dysfunction
• concomitant use of drugs that affect thyroidal axis interactions