The metabolic actions of thyroid hormone and leptin: a mandatory interplay or not?

Excerpt

The report by P. Cettour-Rose and colleagues on the metabolic consequences of hypothyroidism in rats and their partial correction by central leptin infusion in this month’s issue of Diabetologia [1] represents a further step in highlighting the importance of both central and peripheral interplay between thyroid hormones and leptin in the control of energy metabolism. The authors measured whole-body glucose turnover by euglycaemic–hyperinsulinaemic clamp and tissue-specific glucose utilisation in propylthiouracil-induced hypothyroid rats using the labelled 2-deoxy-d-glucose technique. Leptin was infused intracerebroventricularly (i.c.v.) after the induction of hypothyroidism in a group of rats. One of the leptin-infused hypothyroid groups was additionally infused subcutaneously with reverse T3 (rT3) to inhibit type 2 deiodinase activity. Leptin-infused rats treated with rT3 were therefore deficient in both type 1 and type 2 deiodinase activities. In this paper the authors further confirm the presence of decreased overall glucose turnover and glucose utilisation in skeletal muscle and adipose tissue of hypothyroid rats, together with a reduction in leptin and an increase in resistin mRNA expression. Leptin infusion i.c.v. in hypothyroid rats partially restores the impairment in glucose metabolism, and this seems to occur independently of T3 levels and resistin expression. The effect of i.c.v. leptin also resulted in increased expression of carnitine palmitoyl transferases, decreased plasma NEFA levels and reduced muscle triglyceride content. The authors conclude that increased glucose–fatty acid competition may explain the decreased insulin responsiveness in hypothyroidism, and that this phenomenon is partly alleviated by i.c.v. leptin administration. …