Although the goitre seems to be well defined at least from clinical point of view, it is virtually impossible to find the precise definition of the opposite side of what should be the normal thyroid as concerns its size, histological structure and namely the level of growth stimulation by the external factors (such as TSH) and intrinsic tissue growth factors. Theoretically, the normal thyroid should be able to cover the requirements of the organism for the hormone in a reasonably large range without being stimulated to grow by any external or internal factors. So far, the search for normal thyroid has been conducted by several ways: 1. by post mortem thyroid weight, 2. by palpation, 3. by ultrasound. As based on post mortem thyroid weight, until the middle of this century a typical thyroid gland was considered to be about 20-25 g with the accepted upper normal size of 30 g, while more recent studies in iodine replete population have reported mean weights of about 10 g and an upper normal size 20 g. According to several classifications for thyroid palpation the unpalpable thyroid should be allotted to the Grade 0 which is defined as "normal gland", "no goiter at all", "absence of goiter" etc. The first recommendation of normal thyroid volume for children and adolescents as estimated by ultrasound has been developed by Gutekunst and Teichert (1994). However, this was later challenged by the findings of considerable number of thyroid volumes which were higher than the upper limit of that recommendation as found in the countries with satisfactory values of urinary iodine (Delange et al. 1997). Nevertheless, recently it appeared that about 10-15 percent of adolescent thyroids show increased thyroid growth rate which significantly differs from the majority and which might be related to different tuning of molecular growth mechanism presumably of genetic origin (Tajtakova et al. 1998; Langer et al., in press). From, this follows that a certain number of enlarged thyroids apparently should not be included into a normal range.