The thyroid is a butterfly-shaped endocrine gland located below the Adam's apple in the lower part of the neck that wraps around the windpipe (trachea).
It produces hormones that help the body use energy, maintain body temperature, and keep the heart, brain, muscles, and other organs functioning properly.
The thyroid gland releases triiodothyronine (T3) and thyroxine (T4).
Thyroxine (T4) is a hormone that contains iodine. It is a derivative of the amino acid tyrosine. T4 has a number of effects in the body including on:
- Metabolism of fats, carbohydrates, proteins, electrolytes, and water
- Metabolic rate (oxygen consumption)
- Growth and development
- Resistance to infection
- Vitamin requirements
- Reproduction
Triiodothyronine (T3) is a second thyroid hormone produced by the thyroid gland, but also in other tissues through deiodination (enzymatic conversion) of T4. The effects of T3 on the body include:
- Playing a role in metabolic rate
- Maintaining bone health
- Maintaining muscle control
- Preserving brain function and development
- Maintaining heart and digestive functions
What Are Common Thyroid Disorders?
Hypothyroidism (underactive thyroid) occurs when a person produces insufficient amounts of thyroid hormone.
Autoimmune thyroid disease (called Hashimoto’s disease or Hashimoto's thyroiditis) is a form of hypothyroidism. Myxedema coma is a severe form of hypothyroidism.
Common symptoms of hypothyroidism may include:
- Loss of energy
- Fatigue/lethargy
- Weakness in extremities
- Weight gain
- Decreased appetite
- Sleepiness
- Muscle pain
- Joint pain
- Dry skin
- Hair loss
- Depression
- Cold intolerance
- Impaired memory/forgetfulness
- Mental impairment
- Mood swings
- Difficulty concentrating
- Constipation
- Fertility problems
- Menstrual disturbances
- Decreased sweating
- Numbness and tingling and nerve entrapment syndromes
- Decreased hearing
- Blurred vision
- The feeling of fullness in the throat
- Hoarseness
- Nail problems
The opposite of hypothyroidism is hyperthyroidism (also called overactive thyroid), a condition in which the body produces too much thyroid hormone and increases the body’s metabolism.
Some people with hyperthyroidism may have no symptoms. When symptoms of hyperthyroidism occur, they may include:
- Anxiety
- Trouble sleeping
- Irritability
- Weakness
- Hand tremors
- Sweating more than usual
- Fast or uneven heartbeats
- Heat intolerance
- Tiredness/fatigue
- Weight loss even with a normal or increased appetite
- Frequent bowel movements
- Swelling in the neck (goiter)
- Bulging eyes (if hyperthyroidism is caused by Graves' disease)
- Irregular or missed menstrual periods, which may be associated with infertility
- Breast tissue growth in men
- Erectile dysfunction (impotence)
- Atrial fibrillation, chest pain, and rarely, heart failure if untreated
From 
Healthy Resources
Featured Centers
Health Solutions From Our Sponsors
https://www.uptodate.com/contents/hypothyroidism-underactive-thyroid-the-basics?search=Hypothyroidism&source=search_result&selectedTitle=1~150&usage_type=default&display_rank=1
https://www.uptodate.com/contents/hyperthyroidism-overactive-thyroid-beyond-the-basics?search=Hyperthyroidism&topicRef=15437&source=see_link