Thyroid Hormones

Thyroid Hormones

Thyroid Stimulating Hormone
Thyroid-stimulating hormone (also known as TSH or thyrotropin) is a hormone produced by thyrotropes in the anterior pituitary gland which controls the endocrine function of the thyroid gland.

The hypothalamus produces thyrotropin-releasing hormone (TRH) which stimulates the pituitary gland to release TSH. TSH stimulates the thyroid gland to secrete the hormones thyroxine (T4) and triiodothyronine (T3). The production of TSH is inhibited by the production of somatostatin by the hypothalamus. T3 and T4 also inhibit TSH production and secretion, creating a regulatory negative feedback loop.

The thyroid hormones, thyroxine (T4)* and triiodothyronine (T3)*, are tyrosine-based hormones produced by the thyroid gland. They increase the BMR, affect protein synthesis and increase the body’s sensitivity to catecholamines (such as adrenaline). An important component in the synthesis is iodine.

*T3T4

The major form of thyroid hormone in the blood is thyroxine (T4). This is converted to the active T3 within cells by an enzyme called deiodinase.

Most of the thyroid hormone circulating in the blood is bound to transport proteins: thyroid binding globulin (TBG); Thyroid binding prealbumin (TBPA); this protein is also responsible for the transport of retinol, and so now has the preferred name of transthyretin (TTR); albumin

Only a very small fraction of the circulating hormone is free (unbound); T4 0.03% and T3 0.3%. This free fraction is biologically active, hence measuring concentrations of free thyroid hormones is of great diagnostic value. These values are referred to as fT4 and fT3. Another critical diagnostic tool is the amount of TSH that is present; from these measurements you can determine whether someone is suffering from hypothyroidism or hyperthyroidism.

When thyroid hormone is bound, it is not active, the amount of free T3/T4 is what is important. For this reason, measuring total thyroxine in the blood can be misleading.

The thyroid hormones are essential to proper development and differentiation of all cells of human body. To various extent they regulate protein, fat and carbohydrate metabolism. However, the most pronounced impact is on utilisation of energetic compounds by human cells.

Thyrotoxicosis or hyperthyroidism is the clinical syndrome caused by an excess of circulating free thyroxine and free triiodothyronine, or both. It is a common disorder and affects approximately 2% of women and 0.2% of men.

Parathyroid Hormone
Parathyroid hormone (PTH) is secreted by the parathyroid glands as a polypeptide containing 84 amino acids.

PTH acts to increase the concentration of calcium in the blood. It does this in three ways. It enhances the release of calcium from the large reservoir contained in the bones; it enhances reabsorption of calcium from renal tubules; and it enhances the absorption of calcium in the intestine (by increasing the production of 1,25-hydroxyvitamin D).

PTH also acts to decrease the concentration of phosphate in the blood, primarily by reducing reabsorption in the proximal tubules of the kidney.

Increased calcium concentration in the blood acts (via feedback inhibition) to decrease PTH secretion by the parathyroid glands. This is achieved by the activation of calcium-sensing receptors located on parathyroid cells.

Excessive PTH secretion is known as hyperparathyroidism, and is often the result of a benign parathyroid tumour (primary hyperparathyroidism) that loses its sensitivity to circulating calcium levels. In chronic renal failure secondary hyperparathyroidism can result.

Insufficient PTH secretion is known as hypoparathyroidism, and is commonly caused by surgical misadventure, autoimmune disorder, or inborn errors of metabolism.

PTH can be measured in the blood in several different forms: intact PTH; N-terminal PTH; mid-molecule PTH, and C-terminal PTH, and different tests are used in different clinical situations.