Increased 5'-iodothyronine deiodinase activity is a maternal adaptive mechanism in response to protein restriction during lactation

We have shown that protein restriction during lactation is associated with higher levels of serum and milk tri-iodothyronine (T(3)) with lower serum thyroxine (T(4)), suggesting an increased T(4) to T(3) conversion. To investigate this hypothesis, the activity of type 1 (D1) and/or type 2 (D2) iodothyronine deiodinases was evaluated on days 4, 12 and 21 of lactation in several tIssues of dams fed an 8% protein-restricted (PR) diet and controls fed a 23% protein diet. Serum TSH, T(3) and T(4) were measured by radioimmunoassay. Deiodinase activity was determined by the release of (125)I from (125)I-reverse T(3), under specific conditions for D1 or D2. PR dams had a transitory reduction in liver D1 activity (P