Pharmacokinetics and pharmacodynamics of oxazepam and metabolism of paracetamol in severe hypothyroidism

1. The effect of severe hypothyroidism on the pharmacokinetics and pharmacodynamics of oxazepam 15 mg given orally (n = 10) and the metabolism of paracetamol 750 mg given intravenously (n = 8) was investigated before and after treatment with levothyroxine. 2. The median total and unbound clearance of oxazepam increased significantly during the study period from 0.78 ml min-1 kg-1 (0.40-1.25) to 1.22 ml min-1 kg-1 (0.66-1.94) and from 9.3 ml min-1 kg-1 (5.2-14.2) to 15.9 ml min-1 kg-1 (7.8-21.8), respectively (P less than 0.01). 3. The elimination half-life of oxazepam was prolonged by hypothyroidism to a median (range) value of 9.3 h (5.4-21.9) compared with 7.5 h (4.8-10.5) in the euthyroid state (P less than 0.05). 4. Hypothyroidism did not affect the protein binding of oxazepam; median values of the free percentage being 8.2% as compared with 7.7% when euthyroid. 5. The median (range) clearance of paracetamol under hypothyroid conditions was 3.12 ml min-1 kg-1 (1.64-4.40) and 4.70 ml min-1 kg-1 (3.18-5.70) following replacement therapy (P less than 0.01). This increase was associated with a comparable increase in the partial clearance to the glucuronide metabolite: 1.86 ml min-1 kg-1 to 2.70 ml min-1 kg-1. 6. Hypothyroidism was associated with decreased performance in a finger tapping test that was exacerbated by oxazepam. When the patients were euthyroid oxazepam did not produce any effect.