Thiopental is almost completely metabolised in the body with a very small percentage excreted unchanged in urine. Ring desulfuration to the corresponding oxibarbiturate, pentobarbital, retains pharmacological activity. This active metabolite represents 3.3 to 3.6% of an anaesthetic dose and 10% of the parent concentration in the blood of patients receiving high dose thiopental therapy. Both thiopental and pentobarbital undergo oxidation on the terminal carbon atom of the 1-methyl-butyl side chain in the course of metabolic transformation and leads to thiopental carboxylic acid and pentobarbital carboxylic acid, which are pharmacologically inactive. A second metabolic pathway leads to thiopental alcohol and pentobarbital alcohol (metabolites without activity) by hydroxylation in position 3 of the methyl-butyl side chain. Oxidation to the thiopental carboxilic acid is the major step of detoxification in humans, and the urinary excretion of this product accounted for about 10 to 25% of the administered doses. It seems that the main determining factor in the metabolism rate is not the activity of the microcomal enzyme systems but the slow thiopental release from fat deposits.
Pharmacodynamics and pharmacokinetics of thiopental. Russo H, Bressolle F. Clin Pharmacokinet. 1998 Aug;35(2):95-134

The isoenzymes of thiopental metabolism have not been identified, but barbiturates are substrates of cytochrome P450 (CYP), specifically the 2B and 2C families, in rats. CYP3A3 and CYP3A4 can be induced by barbiturates in humans. Pentobarbital is formed to a significant extent when thiopental is given at high doses for several days. Pentobarbital is a potent inducer of hepatic microsomal drug metabolism. The increase in pentobarbital clearance (from 0.047 to 0.079 L/h/kg) by autoinduction in patients treated for reduction of intracranial pressure, became apparent after 6 to 7 days of drug exposure. However, for pentobarbital as a metabolite, concentrations are far below the therapeutic range (15 to 40 mg/L). Pretreatment with enzyme inducer (phenobarbital) or enzyme inhibitor (ethionine) did not modify the sleeptime after thiopental injection in rats. Thus, thiopental action is not limited by metabolism breakdown. Increasing the metabolism of thiopental by phenobarbital injection (pretreatment for 8 days) reduced the period of depression in cattle, but had no effect on the duration of surgical anaesthesia.
Pharmacodynamics and pharmacokinetics of thiopental. Russo H, Bressolle F. Clin Pharmacokinet. 1998 Aug;35(2):95-134