The kinetics of thyroid hormone transporters and their role in non-thyroidal illness and starvation

Kinetic tracer studies show that thyroid hormones are transported into target tissues by stereospecific, high-affinity, low-capacity transporters, both in animals and humans. The Kd of binding to the transporter varies within the nanomolar range. The different thyroid hormones (T4, T3, and rT3) are transported via different transporters, except in the pituitary, where they share the same transporter. The molecular mass of the transport proteins varies between 52 and 65 kDa. The transport mechanisms are dependent on the energy charge of the cell and – often – the sodium gradient over the plasma membrane. A relationship exists with the transport systems of the aromatic amino acids. In non-thyroidal illness and starvation T4 transport into T3-producing tissues is decreased, resulting in a low plasma T3 concentration, by some considered to be an energy saving mechanism in situations of stress.