The bi-lobed, butterfly-shaped thyroid gland is situated on the neck just below the larynx or voicebox. Each lobe is made up of numerous follicles composed of an outer layer of follicular cells and an inner portion containing colloid which consists mainly of protein called thyroglobulin. In between the follicles lie the parafollicular or C cells which secrete calcitonin, a hormone which is thought to play a role in balancing the levels of calcium in the body. The primary hormones produced by the thyroid gland and the ones that are out of balance in thyroid disorders are thyroxine (T4) and triiodothyronine (T3).
Why are Thyroid Hormones Important?
Thyroid hormones influence a number of the body’s important metabolic processes. They increase basal metabolic rate and the sensitivity of the cardiovascular and central nervous system to catecholamines. Catecholamines are important hormones or neurotransmitters which speed up or decrease body functions. They include epinephrine or adrenaline, norepinephrine or noradrenaline and dopamine.
Thus the thyroid gland affects the signals that the brain gives the body as well as cardiac output and heart rate. Additionally thyroid hormones are essential for normal growth and intellectual development in infants and children.
How are Thyroid Hormones Synthesized?
T4 and T3 are synthesized in the thyroid follicles, using dietary iodine and iodide from the blood. Dietary iodine is found in plant based foods, seafood, and iodized salt. Thyroglobulin, which is found in the inner follicles of the thyroid gland is a glycoprotein containing several tyrosyl residues. These tyrosyl residues have iodine added to them to form mono-iodotyrosine (MIT) and di-iodotyrosine (DIT). Iodotyrosol residues are then coupled to form T4 (DIT + DIT) and T3 (MIT+DIT). Therefore T4 is so named because it has four iodine residues attached to it while T3 has three.
The thyroid hormones still incorporated in the thyroglobulin of the thyroid glands are stored in colloid follicles. Colloid follicles fuse with lysosomes, which contain cleaving enzymes and detachment from the follicles brings about the release of T4 and T3 hormones. More T4 than T3 is produced by the thyroid gland, the majority of T3 being formed by deiodination (loss of one iodine residue) of T4 in the blood.
The Brain Influences Thyroid Gland Activity
The synthesis of thyroid hormones is controlled by the brain. The thyroid gland is instructed to produce its hormones by thyroid stimulating hormone (TSH) secreted by the pituitary gland. The pituitary gland is influenced by the hormone tripeptide thyroid releasing hormone (TRH) sent by the hypothalamus portion of the brain. The thyroid hormones themselves exert negative feedback at the level of the pituitary and possibly the hypothalamus, which means that a higher concentration of thyroid hormones in the blood will signal the brain to stop producing thyroid gland stimulating hormones.
