The Kocher-Debre-Semelaigne syndrome (KDSS) is a rare disease that clinically manifests as hypothyroidism and muscle pseudohypertrophy of the trunk and extremities. KDSS occurs mainly in countries where there is no screening for congenital hypothyroidism; however, this syndrome can develop when there is acquired hypothyroidism. In this syndrome, muscle hypertrophy is false (pseudohypertrophy); it is not accompanied by an increase in muscle strength but, on the contrary, leads to a decrease in muscle strength and the development of muscle hypotension. Increased levels of creatinine phosphokinase (CPK) and lactate dehydrogenase (LDH) are the characteristic laboratory sign of myopathy in this syndrome. The signs of hypothyroidism and muscle pseudohypertrophy are reduced by levothyroxine therapy. Indicators of the final height in the case of delayed treatment may remain poor. The clinical signs characteristic of the disease, especially in the presence of a nonspecific clinical picture of hypothyroidism, enable an early diagnosis and timely treatment, which emphasizes the importance of doctors’ awareness of this syndrome.