Signs of an underactive thyroid may include: 

• lower energy levels;
• weight gain;
• sensitivity to cold;
• constipation;
• muscle weakness and cramps;
• dry and scaly skin;
• puffy face;
• brittle nails;
• thinning hair;
• depression;
• loss of libido;
• irregular or heavy periods;
• memory problems (in the elderly);
• slower growth and development;
• early puberty;
• jaundice (in infants);
• frequent choking (in infants); and
• large and protruding tongue (in infants). 

An underactive thyroid does not produce enough thyroid hormone. Thyroid hormones travel through the bloodstream and affect nearly every part of the body, from the heart and brain to the muscles and skin. The thyroid uses fats and carbohydrates and helps regulate protein production, metabolism, body temperature and heart rate. An underactive thyroid causes body processes to slow down, leading to weakness, fatigue and complications that may mimic other health conditions.