BIOCHEMISTRY - Main Regulators and Biological Fluids of the Human Body - 2016

2. HORMONES

2.6. Amino Acid-Derived Hormones

1. THYROID Hormones are derivatives of The amino acid Tyrosine. The most active among them are triiodothyronine (T3) and tetraiodothyronine (T4). The synthesis of thyroid hormones proceeds According to the following scheme:

Class="center">

In The Thyroid Gland, all these hormones are bound to Proteins. Upon dissociation, they enter the bloodstream, bind to Plasma Proteins, and are delivered to target Organs and Tissues. T3 and T4 hormones elevate basal metabolic rate, enhance the Catabolism of proteins, fats, and CARBOHYDRATES, and increase thermogenesis.

Excessive production of thyroid hormones (hyperthyroidism) leads to Graves' disease, characterized by elevated gas exchange and heat production, increased urinary nitrogen excretion, heightened oxidative processes, exophthalmos, and general emaciation. Conversely, hormone deficiency (hypothyroidism) in children results in dwarfism, structural body deformities, and cretinism, whereas in adults it leads to Myxedema. This condition is marked by edema, tissue Water retention, lethargy, and obesity. Surgical removal of the thyroid gland in animals causes a drastic drop in METABOLISM, which returns to normal upon exogenous hormone administration. It should be noted that excessive external administration of hormones induces phenomena characteristic of an elevated metabolic rate; moreover, the effects of these preparations do not manifest immediately, but rather after several hours. This indicates that hormones influence tissue oxidative processes not directly, but through indirect pathways.

A direct link has been established between the hormonal function of the thyroid gland and the excitation or inhibition of the Central Nervous system. Stimulation of the nerves innervating the thyroid gland leads to an upregulation of its Functions. At the same time, the regulatory effect of hormones on metabolism undeniably occurs with the participation of the CNS.

The Synthesis and Secretion of iodothyronines are regulated by the hypothalamic-pituitary system via a negative feedback loop.

The Hypothalamus produces thyrotropin-releasing hormone (TRH), which enhances the synthesis of thyroid-stimulating hormone (TSH) by the Pituitary Gland. TSH, in turn, stimulates thyroid function and The production of T3 and T4 hormones. When thyroid hormone levels in the Blood are elevated, they inhibit the pituitary production of TSH and hypothalamic production of TRH. The hypothalamus also synthesizes Somatostatin (GH-inhibiting hormone), which likewise inhibits the release of thyroid-stimulating hormone (Fig. 9).

Fig. 9. Regulation scheme of iodothyronine synthesis and secretion

2. Hormones of The adrenal medulla - catecholamines.

In humans, the Adrenal Glands are paired organs consisting of a right gland and a slightly smaller left gland in terms of weight and size. The adrenal glands are differentiated into two layers: the medulla and the cortex. Each layer consists of distinct tissue and performs its own internal secretory function.

The medullary tissue is capable of producing two hormones: adrenaline and noradrenaline. In humans, 1 g of adrenal tissue contains 0.49 mg of adrenaline and 0.09 mg of noradrenaline. They are derivatives of the amino acid phenylalanine. Upon entering the bloodstream, adrenaline and noradrenaline exist in a free or protein-bound state (acting as their transport and reserve forms). When lactic acid accumulates in the blood, catecholamines dissociate from proteins and exert their effects. For instance, adrenaline raises blood pressure and Heart rate, enhances gas exchange and heat production, stimulates Glycogenolysis in the Liver by activating adenylate cyclase and phosphorylase Enzymes, and elevates blood glucose levels. In Muscles, adrenaline enhances Glycogen breakdown and promotes lactic acid formation, while in adipose tissue it accelerates lipidolysis, releasing free Fatty acids into the bloodstream. Conversely, adrenaline inhibits gastrointestinal tract function. Thus, it can be said that adrenaline helps you run, think, and breathe, but not digest food. Noradrenaline is similar in action to adrenaline, but weaker.

An increase in catecholamine secretion occurs during drops in blood glucose and oxygen levels (Hypoxia), during intensive muscular activity, as well as in response to pain and strong emotions.