The uptake of T3 and T4 into the cells is by a specific carrier-mediated uptake across the plasma membrane. The process is both energy and sodium dependent, as the plasma membrane transporter is a Na+ dependent organic anion transporter. Diffusion is a process ruled out for crossing the plasma membrane in this case as the mean radii of T3 and T4 are both greater than the pore radius of plasma membrane.
The rate of uptake into cells determines the rate at which T3 is produced from T4 this in turn causes effects such as a change in oxygen consumption and a change in the rate of burning of proteins, carbohydrates and lipids.
If the amount of ATP in the body tissue decreases (i.e. a decrease in the bodies calorie uptake) there is a decrease in in cellular uptake of T3 and T4. Two days of fasting can inhibit T3 uptake into some cells by 40%. Uptake can also be inhibited by some substances such as fatty acids.