Decrease of blood glucose, liver glycogen, and insulin and changes in glucose tolerance in suckling rats treated neonatally with high L-thyroxine doses.

The neo-T4 syndrome was induced by sc administration of high doses (30 microgram) of L-T4 (T4) during 5 consecutive days in the early postnatal period. Rats were injected starting on the 3rd or 6th day of life. Blood glucose and insulin levels as well as liver glycogen levels were determined between the 5th and 20th days of life. This covers the critical period for brain development (the 7th to 12th day) during which most myelination takes place. Blood glucose and insulin and liver glycogen started to decrease after the second injection of T4 and remained quite low until 2 days after the last injection. Glucose tolerance testing was performed on 8- and 12-day-old neo-T4 rats; treatment was started at 3 days of age. After 70 min of fasting, 200 mg glucose/100 g BW were injected ip. Low plasma insulin levels were found after a glucose load. Plasma ketone bodies were higher in neo-T4 rats than in controls after 4 or 5 days of treatment. The rats appeared to be in a severe catabolic state and were using up their reserves. These alterations in carbohydrate metabolism in suckling rats should be considered when interpreting the brain damage produced by large doses of T4 during the early neonatal period.