“Sick-euthyroid syndrome” is the term used to describe changes in serum thyroid hormone and thyroid stimulating hormone (TSH) levels with acute illness not caused by an intrinsic abnormality of thyroid function. Usual symptoms include fatigue and weakness, weight gain, puffy face, cold intolerance, constipation and bradycardia. It may result in infertility in adults. Blood tests of thyroid-stimulating hormone (TSH) level and T4 (thyroxine) level are very important for the diagnosis of euthyroid sick syndrome.