Thyroid means relating to the thyroid gland — the small, butterfly-shaped endocrine gland in the front of the neck. Thyroid hormones regulate the body’s metabolic rate: how fast cells burn energy, how the heart beats, how warm the body runs. In nutrition the thyroid matters because it needs iodine to make its hormones; iodine deficiency causes goitre and, in pregnancy, irreversible damage to the baby’s brain development — which is why iodised salt programmes exist. Both an overactive and an underactive thyroid disturb weight, energy, and digestion. Thyroid function is assessed by blood tests, and disorders are common, treatable, and often first suspected from their metabolic symptoms.