In the theoretical part of my thesis, I describe the thyroid, its importance, anatomy, creation of hormones and their regulation. There is also summarized the knowledge of external environment influences on the thyroid such as nutrition, climatic conditions or seasons and internal factors influences, mainly breed, efficiency and age. The practical part is focused on the evaluation of the thyroid activity of free-range ewes and lambs in the spring and autumn of 2013. The thesis also deals with the evaluation of possible relevance between the content of thyroid hormones in the blood serum and the physiological state of the sheep. Thanks to the correlation between TSH and thyroid hormones, the proper regulation of the thyroid of ewes and lambs was confirmed. The thyroid activity was higher in the spring than in the autumn. The hypothyroidism in the autumn was not proved because no referential values of TSH are set. In the autumn there was a lower level of glucose and cholesterol in the blood of the ewes determined which gives evidence of a lower energy income from the feed in the autumn. Also the urea level which is closely connected to the energy shortage, starvation, and high income of N substances in the feed dosage was in the autumn twice as high as in the spring. There was a positive correlation determined between the higher urea concentration and the higher thyroid activity. Owing to the change of the feed content, the TSH started to stimulate the thyroid to a higher thyroid hormones production and thus to its higher activity.