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

Abstract

We have shown that protein restriction during lactation is associated with higher levels of serum and milk tri-iodothyronine (T3) with lower serum thyroxine (T4), suggesting an increased T4 to T3 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, T3 and T4 were measured by radioimmunoassay. Deiodinase activity was determined by the release of 125I from 125I-reverse T3, under specific conditions for D1 or D2. PR dams had a transitory reduction in liver D1 activity (P<0.05) on day 12, and a small increase in thyroid D1 on day 12 followed by a small decrease on day 21. However, thyroid D2 activity was higher than controls (P<0.05) during the whole of the lactation period. Mammary gland D1 and D2 activities were lower on day 4 of lactation in PR dams (P<0.05), and D2 was higher on day 21 (P<0.05). Potentially, a lower conversion of T3 to di-iodothyronine in the mammary glands of PR dams at the beginning of lactation may serve to provide more T3 through the milk. Brown adipose tissue (BAT) D2 activity was higher (P<0.05) in PR dams during all periods of lactation. PR dams showed higher skeletal muscle D1 activity only at the end of lactation, but no changes in D2 activity. Higher pituitary D1 and D2 activities in the PR group (P<0.05) at the end of lactation could have contributed to the lower serum TSH. These data suggest that the higher thyroid and BAT D2 activity during the whole of lactation and skeletal muscle D1 activity at the end of lactation may contribute to the higher serum T3 in PR dams.