The thyroid is a butterfly-shaped endocrine gland that functions to produce hormones, which help the body use energy, maintain body temperature, and keep the heart, brain, muscles, and other organs functioning properly. It is located below the Adam's apple in the lower part of the neck that wraps around the windpipe (trachea).
The thyroid gland releases the hormones thyroxine (T4) and triiodothyronine (T3).
- 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)
- Vitamin requirements
- Resistance to infection
- Growth and development
- Reproduction
- Triiodothyronine (T3) is a second thyroid hormone produced by the thyroid gland, but it is also produced in other tissues through deiodination (enzymatic conversion) of T4
- The effects of T3 on the body include:
- Plays a role in metabolic rate
- Helps maintain heart and digestive functions
- Helps maintain bone health
- Helps maintain muscle control
- Helps preserve brain function and development
- The effects of T3 on the body include:
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.
Symptoms of hypothyroidism may include:
- Fatigue/lethargy
- Loss of energy
- Weakness in extremities
- Weight gain
- Decreased appetite
- Sleepiness
- Joint pain
- Muscle pain
- Dry skin
- Hair loss
- Depression
- Intolerance to cold
- Constipation
- Memory problems
- Mental impairment
- Mood swings
- Difficulty concentrating
- Fertility problems
- Menstrual irregularities
- Decreased sweating
- Numbness and tingling and nerve entrapment syndromes
- Hearing loss
- Blurred vision
- 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.
Hyperthyroidism may not cause any symptoms. When symptoms do occur, they can include:
- Anxiety
- Irritability
- Problems sleeping
- Weakness
- Sweating more than usual
- Hand tremors
- Fast or uneven heartbeats
- Heat intolerance
- Tiredness
- Weight loss even with a normal or increased appetite
- Swelling in the neck (goiter)
- Frequent bowel movements
- Irregular or missed menstrual periods, which may be associated with infertility
- Breast tissue growth in men
- Erectile dysfunction (impotence)
- Bulging eyes (when caused by Graves' disease)
- Atrial fibrillation, chest pain, and rarely, heart failure if untreated
From 
https://www.hormone.org/your-health-and-hormones/glands-and-hormones-a-to-z/hormones/thyroid-hormones
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