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== Transport of Triiodothyronine ==
The T<sub>3</sub> (and T<sub>4</sub>) bind to nuclear receptors, thyroid receptors. However, T<sub>3</sub> (and T<sub>4</sub>) are not very lipophilic and as a result, are unable to pass through the phospholipid bilayers. They therefore have specific transport proteins on the cell membranes of the effector organs which allow the T<sub>3</sub> and T<sub>4</sub> to pass into the cells. The thyroid receptors bind to response elements in gene promoters and thus enabling them to activate or inhibit transcription. The sensitivity of a tissue to T<sub>3</sub> is modulated through the thyroid receptors.