The thyroid gland produces and secretes the thyroid hormones triiodothyronine and thyroxine. These hormones circulate in the bloodstream, controlling the overall pace of body functions. Abnormally increased or decreased thyroid hormone production--known as hyperthyroidism and hypothyroidism, respectively--manifests with symptom-producing metabolic disturbances in many body tissues. The nature of the symptoms helps pinpoint whether the defect in thyroid function reflects an overactive or underactive thyroid gland.
Body Temperature Abnormalities
By controlling the metabolic rate of the body, the thyroid gland affects body temperature, explains the medical reference text "Harrison's Principles of Internal Medicine." Abnormally elevated thyroid hormone levels increase body temperature, causing heat intolerance among people with hyperthyroidism. Conversely, people with abnormally low thyroid hormone levels often feel unusually cold. With either thyroid condition, correction of the thyroid hormones to normal levels results in resolution of body temperature abnormalities.
Mood and Mental Changes
The thyroid hormones influence the pace of brain functions. The decreased metabolic rate of hypothyroidism often causes irritability, depression and mental slowness, reports the American Thyroid Association. In contrast, the increased metabolic rate associated with hyperthyroidism typically causes anxiety, nervousness, rapid mood swings and irritability. Markedly elevated or depressed levels of thyroid hormones can cause profoundly disturbed thinking, which may mimic a psychiatric disorder.
Weight Change and Digestive Disturbances
The elevated metabolic rate associated with hyperthyroidism causes an increase in the number of calories used each day. Despite an increased appetite, people with hyperthyroidism often lose weight, reports the Merck Manual for Healthcare Professionals. Increased stool frequency or diarrhea also commonly occurs. Among people with hypothyroidism, the opposite effects occur. A decreased metabolic rate and fluid retention typically leads to weight gain. Decreased activity of the digestive tract also commonly causes constipation among people with an underactive thyroid gland.
Altered Sleep Patterns
People with abnormally low levels of thyroid hormones characteristically experience daytime sleepiness and an increased amount of sleep time, according to the American Association of Clinical Endocrinologists. In contrast, people with an overabundance of thyroid hormones commonly report insomnia.
Muscle Weakness and Fatigue
Abnormally elevated or depressed thyroid hormone levels commonly cause muscle weakness and fatigue, notes the American Thyroid Association. Muscular weakness often manifests as decreased exercise tolerance. Slowed metabolism underlies the fatigue associated with hypothyroidism. Sleeplessness and persistent, excessive metabolic activity provokes the fatigue experienced by people with hyperthyroidism.
Menstrual Irregularity
Thyroid hormone levels influence the female reproductive system. Among women of childbearing age, hyperthyroidism or hypothyroidism can cause menstrual irregularities and impaired fertility, reports the Merck Manual for Healthcare Professionals. Menstruation may stop in some women with defective thyroid hormone production.
References
- "Harrison's Principles of Internal Medicine, 16th Edition"; Dennis L. Kasper, M.D., et al., Editors; 2004
- American Thyroid Association: ATA Hypothyroidism Booklet
- American Thyroid Association: Hyperthyroidism
- Merck Manual for Healthcare Professionals: Hyperthyroidism, Signs and Symptoms
- Merck Manual for Healthcare Professionals: Hypothyroidism, Signs and Symptoms
