The fate of N-methyl-N'-(hydroxymethyl)thiourea in the rat.

1. The fate of N-methyl-N'-(hydroxy[14C]methyl)thiourea (MHT) has been studied in the male Sprague-Dawley rat. The compound is degraded to N-methylthiourea and formaldehyde. 2. N-Methylthiourea is excreted as a urinary metabolite whereas the formaldehyde is not excreted, either in the urine or the expired air, but is metabolized via formate to CO2. 3. At least 50% of an i.p. dose (100 mg/kg) of MHT is excreted unchanged and some of this undergoes hydrolysis within the urine to N-methylthiourea and formaldehyde. Production of formaldehyde leads to the formation of the urinary artefact N-(hydroxymethyl)urea. 4. Tissue-distribution studies with 14C-MHT have shown that radioactivity is selectively associated with the thyroid gland. A preliminary investigation has indicated that MHT has anti-thyroid hormone activity as it lowers the thyroxine in rat serum.