Читать книгу The Healthy Thyroid: What you can do to prevent and alleviate thyroid imbalance - Patsy Westcott - Страница 16

The whole endocrine system is controlled by a series of ‘feedback loops’, which slow or stop a gland from working when enough hormone has been produced, and turn it back on again when more is needed – like a central-heating thermostat (see Figure 2.2). If the blood levels of any of the essential chemicals are low, special sensory cells are able to pick up a signal that

Fig 2.1 The endocrine system

Figure 2.2 The pituitary gland and the hypothalamus in the brain work together to produce a hormone that stimulates the thyroid. The thyroid gland draws iodine from the blood in order to make T3 and T4. Sensors in the TSH-secreting cells of the pituitary detect rising levels of thyroid hormones and quell further secretion. When levels fall, the pituitary releases more TSH, which stimulates the thyroid to start making more hormones.

prompts them to release hormone. This hormone, in turn, acts on other cells to release more of the needed chemical into the bloodstream. When enough chemical has been produced, the sensory cells switch the system off, which stops further hormone release. In this way, the body’s chemical balance is constantly maintained.

The system is exquisitely sensitive: food, exercise, stress, illness, changes in body chemistry such as a shortage or excess of certain nutrients, pregnancy, ageing, even the time of day or year, can affect the balancing mechanism and, with it, the amount of hormones our glands secrete.

Most hormones act only on specific tissues and not all the cells in the body. They do this by latching on to structures called ‘receptors’, which lie studded about the surface of or within cells, rather like a key fits into a lock. This enables hormones to be transported around the bloodstream to specific locations. Receptors are also important because, as we shall see, if the wrong chemical – such as an autoimmune antibody – attaches itself to a receptor, like a thief using a master key to get into your house, it can cause havoc and destruction.