Читать книгу Essential Endocrinology and Diabetes - Richard I. G. Holt - Страница 30

Superimposed on the regulatory systems described above, many of the body’s activities show additional periodic or cyclical changes (Box 1.5). Control of these rhythms commonly arises from the nervous system via the hypothalamus. Some appear independent of the environment, whereas others are coordinated and ‘entrained’ by external cues (e.g. the 24‐h light/dark cycle, which becomes temporarily disrupted in jetlag). Cortisol secretion is maximal between 0400h and 0800h as we awaken and minimal as we retire to bed. In contrast, GH and prolactin are secreted maximally ∼1 h after falling asleep. Clinically, this knowledge is important as investigation must be referenced according to hour‐by‐hour and day‐to‐day variability. Otherwise, such laboratory tests may be invalid or, indeed, misleading.