The term means low thyroid function. Hypothyroidism probably affects hundreds of thousands of people but is often never diagnosed. Clinical signs and symptoms of hypothyroidism include the following (1,2,3):
Low body temperature, dry skin/hair, (red hair is at particular risk for low thyroid), inappropriate weight gain and/or an inability to lose weight, brittle nails, insomnia and/or narcolepsy, poor short-term memory and concentration, fatigue, headaches and migraines, premenstrual syndrome and related problems, menstrual irregularities, depression, hair loss (including outer third of eyebrows), low motivation and ambition, cold hands and feet, fluid retention, dizziness or lightheadedness, irritability, easy bruising, skin problems/infections/acne, infertility, dry eyes/blurred vision, heat and/or cold intolerance, low blood pressure, elevated cholesterol, digestive problems (irritable bowel syndrome, acid indigestion, constipation, etc.), poor coordination, diminished sex drive, reduced or excessive sweating, frequent colds/sore throats, asthma/allergies, slow healing, itchiness, food cravings, recurrent infections, food intolerances, increased susceptibility to substance abuse, anxiety/panic attacks, yellow-orange coloration on skin (particularly palms), yellow bumps on eyelids, slow speech, thickened tongue with scallop-like indentations, fluid in the ears, etc.
Because long-term low thyroid function causes poor circulation and reduces delivery of oxygen to tissue cellular and therefore has an association with heart disease and cancer as well as making you feel miserable, having even a few of these symptoms warrants checking your thyroid function.