Information Living with thyroid disorders About the thyroid What is the thyroid? The thyroid gland is an endocrine gland. It makes hormones, which are chemical ‘messengers’ that travel through your bloodstream to all the cells and tissues in your body and affect how they work. Where is the thyroid gland? Your thyroid gland is in the front of your neck, just below your Adam’s apple (your larynx). It has two lobes, which are on either side of your windpipe. The lobes are joined together by some tissue called the isthmus. In an adult, each lobe is about 5cm (2 inches) long. In children, the lobes are proportionately smaller. They grow as the child grows. What does the thyroid gland do? The thyroid gland makes two hormones that are secreted in the bloodstream: thyroxine, which is also called T4; and triiodothyronine, which is known as T3. T4 is turned into T3 in the body’s cells and tissues. It is the T3, derived from T4 or secreted as T3 from the thyroid gland that affects the activity of your cells and tissues. What do the thyroid hormones do? Thyroid hormones perform a range of vital functions including: controlling the body’s metabolism and so affecting the speed with which your cells work. This in turn can affect heart rate, breathing, circulation and digestion; affecting the body’s growth and therefore playing a vital role in helping children to grow and develop; helping the brain to develop from when the baby is in the womb. How is the thyroid gland controlled? The thyroid gland is controlled by the pituitary gland. This is found in the skull, underneath the brain. It can help to think of the pituitary gland as behaving like a thermostat that regulates the temperature and central heating in a house. The pituitary senses the level of thyroid hormones in the bloodstream, just as the thermostat in your living room senses the temperature. If a thermostat senses that the temperature has fallen below its setting, the central heating switches on. If the thermostat senses that the temperature has risen, the central heating switches off. Similarly, if the level of thyroid hormones drops below normal, the pituitary gland will release a hormone called thyroid-stimulating hormone or TSH. This hormone effectively tells the thyroid gland to release more T4 and T3. If the thyroid hormones rise above the normal level, the pituitary gland will reduce the amount of TSH it releases so that the thyroid gland makes less T4 and T3. How much thyroid hormone do children produce? The amount of thyroid hormones that babies, children and young people produce varies depending on their age and size. Doctors can test whether a thyroid gland is working properly by looking at the level of TSH and the level of thyroid hormone (frequently the amount of free thyroxine) in the blood. Reference ranges are used to interpret the blood test results. Different laboratories may have slightly different reference ranges depending on how the levels of TSH and thyroid hormone are measured. Your doctor will explain the results of the blood test. If your child has a low TSH level in their blood, then they probably have an overactive thyroid gland (producing too much T4/T3). If they have a high level of TSH in their blood, then they probably have an underactive thyroid gland (producing too little T4/T3).