The thyroid gland is a butterfly-shaped endocrine organ that lies just below the Adam’s apple. It produces the hormones triiodothyronine, or T3, and thyroxine, or T4. The hypothalamus and the pituitary gland regulate the synthesis and release of thyroid hormones, which are synthesized mainly from iodine and the amino acid tyrosine. T4 is the primary circulating active thyroid hormone and acts on all cells of the body, regulating development, growth and metabolism.
Iodine
The only use of iodine in humans is for the production of thyroid hormones. The only source of iodine is dietary, and it is most concentrated in marine fish and shellfish. It is also found in water and soils that are near the oceans.
Iodine Deficiency
The World Health Organization reported that thyroid hormone is critical for the development of the brain and central nervous system from the 15th week of gestation to 3 years of age. The most common cause of fetal and neonatal hypothyroidism worldwide is iodine deficiency. Children with neonatal iodine deficiency can develop irreversible mental retardation, hearing impairment and dwarfism. The most severe debilitation is known as cretinism, or severely stunted physical and mental growth, which in 1990 affected more than 10 million children worldwide. In older children and adults, iodine deficiency can cause goiter, which is an enlargement of the thyroid gland. The WHO recommends a dietary intake of 150 milligrams per day of iodine for adolescents and adults to avoid the development of goiter. In the U.S., iodine deficiency disorders have been largely eliminated by the use of iodized salt. In rare cases, inherited defects can cause hypothyroidism, which may also cause mental retardation and goiter.
Hypothyroidism
In populations where iodine deficiency is uncommon, hypothyroidism is primarily caused by an autoimmune disorder called Hashimoto's thyroiditis. A study published in the journal Clinical Endocrinology reported that the incidence of hypothyroidism attributed to disorders other than iodine deficiency is 3.5 per 1,000 in women and 0.6 per 1,000 in men. In addition to goiter, in adults, the symptoms of hypothyroidism may manifest as cold intolerance, muscle cramping, fatigue, depression, weight gain, itchy skin and low heart rate. A study in the journal Thyroid reported that 34 percent of people with Hashimoto's thyroiditis have upper eyelid retraction; however, the eye muscle abnormalities are more subtle than those observed in people with Graves' disease.
Hyperthyroidism
The most common cause of hyperthyroidism, or overactive thyroid, is also an autoimmune disease, known as Graves’ disease. Benign tumors in the pituitary and thyroid and nodules that form in goiters may also cause hyperthyroidism. The study in Clinical Endocrinology reported the incidence of hyperthyroidism in women as 0.8 per 1,000 and in men, the disorder has a negligible incidence. Thyroid hormone increases basal metabolism, and symptoms of excess circulating thyroid hormone include weight loss, hyperactivity, increased heart rate, heat intolerance, tremors and sweating. Also, people with hyperthyroidism may exhibit anxiety, irritability and delirium. The study in the journal Thyroid reported that nearly 40 percent of patients with Graves' disease display significant eye abnormalities, including swelling, redness and protrusion of the eye. The effects on the eyes are caused by self-recognizing antibodies attacking muscle cells surrounding the eye.
Thyroid Cancer
Nodules of the thyroid gland are very common, and the majority are not cancerous and do not cause any symptoms. A review found on the Endocrine Web website said far less than 1 percent of thyroid nodules are cancerous. However, the incidence of thyroid cancer is rising. A study in the journal, Otolaryngologic Clinics in North America said thyroid cancer is the most common endocrine malignancy, and its incidence is increasing in the United States. This study noted that in 2008, 37,340 cases of thyroid cancer were reported in the United States. Fortunately, the Endocrine Web website also reported that the most common forms of thyroid cancer that account for nearly 95 percent of all thyroid cancers are highly curable.
References
- Colorado State: Mechanism of Action and Physiologic Effects of Thyroid Hormones
- World Health Organization: Iodine
- Thyroid Manager: The Iodine Deficiency Disorders
- PubMed: Incidence of Thyroid Disorders in the Community
- PubMed: Eye and Eyelid Abnormalities are Common in Patients with Hashimoto's Thyroiditis
