- Thyroid hormones stimulate bone maturation as a result of ossification and fusion of the growth plates.  In thyroid hormone deficiency, bone age is less than chronologic age.
- Maturation of the CNS is absolutely dependent on thyroid hormone in the perinatal period (after birth).
- Thyroid hormone deficiency causes irreversible mental retardation.  Because there is only a brief perinatal period when thyroid hormone replacement therapy is helpful, screening for neonatal hypothyroidism is mandatory.
- In adulthood, hypothyroidism causes listlessness, slowed speech, somnolence, impaired memory, and decreased mental capacity.
- Symptoms of hypothyroidism include decreased metabolic rate, weight gain, positive nitrogen balance, decreased heat production (sensitivity to cold), decreased cardiac output (decreased heart rate and stroke volume), hypoventilation, lethargy, mental slowness, drooping eyelids (ptosis), myxedema, growth and mental retardation in children, and goiter.
- Causes of hypothyroidism include thyroiditis (autoimmune thyroiditis, and Hashimoto's thyroiditis), surgical removal of the thyroid (thyroidectomy), iodine deficiency, cretinism (a congenital disorder which manifests as mental retardation and growth deficiency), and decreased TRH or TSH.
- TSH levels are increased because of decreased feedback inhibition on the anterior pituitary by low thyroid hormone levels).
- TSH levels are also decreased if the primary defect is in the hypothalamus or anterior pituitary gland.
- Treatment for hypothyroidism is thyroid hormone replacement therapy.