Thyroid hormones (TH) cross the plasma membrane with the help of transporter proteins. As charged amino acid derivatives, TH cannot simply diffuse across a lipid bilayer membrane, despite their notorious hydrophobicity.

How are thyroid hormones transported?

THs are actively transported across tissue barriers, including placenta, and brain blood barrier (BBB), and into target cells. In circulation free THs are present only in minute amounts and mostly are bound to carrier-proteins.

Why is thyroid hormone hydrophobic?

Like cholesterol, steroid hormones are not soluble in water (they are hydrophobic). Because blood is water-based, lipid-derived hormones must travel to their target cell bound to a transport protein.

How is most thyroid hormone transported in the plasma?

The secreted thyroid hormone is largely in the thyroxine (3,3′,5,5′-tetraiodothyronine, T4) precursor form and in smaller quantities in its active form (3,3′,5-triiodothyronine, T3) and is transported in the plasma bound to passive carriers, including albumin, thyroxine-binding globulin, and transthyretin (TTR).

How do the thyroid hormones enter the nucleus of a target cell?

Thyroid hormones enter target cells via specific membrane transporters, such as Mct9. The intracellular concentration of T3 is determined by the relative activities of the deiodinases, Dio 1 and Dio 2. T3 enters the nucleus and binds to nuclear thyroid hormone receptors (THRs) to regulate expression of T3 target genes.

What transports thyroid hormones in the blood?

Transport in the Blood More than 99% of the circulating thyroid hormone is bound to plasma pro- teins but can be liberated with great rapidity for entry into cells.

How do thyroid hormones cross the cell membrane?

Thyroid hormones (TH) cross the plasma membrane with the help of transporter proteins. As charged amino acid derivatives, TH cannot simply diffuse across a lipid bilayer membrane, despite their notorious hydrophobicity.

Is thyroid hormone hydrophobic?

The thyroid hormones are very hydrophobic and those that exhibit biological activity are 3′,5′,3,5-L-tetraiodothyronine (T4), 3′,5,3-L-triiodothyronine (T3), 3′,5′,3-L-triiodothyronine (rT3) and 3,5′,-L-diiothyronine (3,5-T2).

Why is thyroid hormone lipophilic?

Thyroid hormones are therefore highly lipophilic molecules due to the iodinated aromatic rings. In spite of their lipophilicity, the cellular uptake of thyroid hormones is effected by energy dependent, carrier-mediated processes [5]. Moreover, iodothyronines are normal constituents of biological membranes [6].

Are thyroid hormones water soluble?

Thyroid hormones are poorly soluble in water, and more than 99% of the T3 and T4 circulating in blood is bound to carrier proteins. The principle carrier of thyroid hormones is thyroxine-binding globulin, a glycoprotein synthesized in the liver.

How do thyroid hormones exist in blood plasma?

Plasma transport Most of the thyroid hormone circulating in the blood is bound to transport proteins, and only a very small fraction is unbound and biologically active. Therefore, measuring concentrations of free thyroid hormones is important for diagnosis, while measuring total levels can be misleading.

Does albumin transport thyroid hormones?

TBG binds thyroid hormones in circulation. It is one of three transport proteins (along with transthyretin and serum albumin) responsible for carrying the thyroid hormones thyroxine (T₄) and triiodothyronine (T₃) in the bloodstream.

Do thyroid hormones need a transport protein?

These transport proteins deliver the hormones to their target cells and protect them from being chemically altered, inactivated, and eliminated from the body by the liver and kidneys. Some hormone transport proteins in plasma are highly selective, transporting only steroid or only thyroid hormones.

Which transports thyroid hormones into target cells?

Once made, the thyroid gland releases the hormones into the bloodstream where protein chaperones, called thyroid transport proteins, accompany them to target cells in tissues all over the body.

How do hormones reach target cells?

Hormones activate target cells by diffusing through the plasma membrane of the target cells (lipid-soluble hormones) to bind a receptor protein within the cytoplasm of the cell, or by binding a specific receptor protein in the cell membrane of the target cell (water-soluble proteins).

How does T3 and T4 enter target cells?

Although it was originally believed that thyroid hormones enter target cells by passive diffusion, it is now clear that cellular uptake is effected by carrier-mediated processes. Two stereospecific binding sites for each T4 and T3 have been detected in cell membranes and on intact cells from humans and other species.