Summary of Main Physiologic Actions of Thyroid Hormones in Children and Adults
| Target tissue or organ | Action | Effect of excess thyroid hormone |
| All tissues and organs | Overall function and metabolism | Increased basal metabolic rate and thermogenesis; heat intolerance and sweating; weight loss |
| Cardiovascular and respiratory systems | Optimal heart rate, stroke volume, and force of contraction; vascular resistance; optimal respiratory rate | Increased resting heart rate; cardiac arrhythmias; systolic hypertension; increased respiratory rate and shortness of breath |
| Nervous system | Overall development and function | Increased wakefulness; mood instability and nervousness; tremor; proptosis; increased peripheral reflexes; inability to concentrate |
| Gastrointestinal tract | Optimal tone and motility | Increased tone and motility, with hyperdefecation |
| Skin and skin annexes | Stimulation of keratinocytes, fibroblasts, hair erector pili, and other smooth muscle cells; sebaceous gland cells; vascular endothelial cells; cells of hair follicle; and nail-forming cells | Infiltrative dermopathy and acropachy; hair loss; terminal increased nail growth with onycholysis |
| Musculoskeletal system | Optimal growth of skeletal muscles and development of mineral component of bones | Proximal muscle weakness; decreased bone mineral mass; fragility fractures |
| Reproductive apparatus | Optimal regulation of gonadal function in women and men | Oligodysmenorrhea; infertility; miscarriages |
Optimal function of target tissues and organs is associated with normal levels of thyroid hormones.