What is the thyroid?
The thyroid a ‘bowtie-shaped’ gland found at the front of the neck, just below the Adam’s apple. It is responsible for producing thyroid hormones.
The thyroid gland forms part of the endocrine system which produces hormones (chemical messengers) in the body.
What does the thyroid gland do?
The two main hormones that the thyroid produces are thyroxine (T4) and triidothyronine (T3).
T4 and T3 control the body’s metabolism (the rate that chemical reactions occur in the body) amongst many other body processes, including growth in the early years of life.
About 85% of the thyroid hormone released into the bloodstream is T4 and the remaining 15% is T3.
Release of hormones
Thyroid hormones are released as a result of chemical signals from the pituitary gland that are in turn prompted by chemical signals from the hypothalamus. Both of these structures are found within the brain.
Specifically, thyrotropin-releasing hormone (TRH) being released from the hypothalamus causes the pituitary gland to release thyrotropin (TSH) which then causes the thyroid gland to release T4 and T3.
You may read references to ‘free’ T3 and T4. The majority of T3 and T4 found in the bloodstream is bound to a protein called ‘thyroxine binding globulin’ or ‘TBG’.
The bound forms of these hormones are inactive in contrast to the unbound or ‘free’ forms and therefore it is the ‘free’ forms that cause our metabolic processes to happen.
What do T3 and T4 do?
T3 stimulates our body cells causing their metabolic processes to happen. These processes include those responsible for metabolism, growth and mental development, amongst many others.
Every organ in the body that requires T3 to function has the ability to convert T4 to the T3 it needs.
T4 is converted to T3 by ‘deiodinase’ enzymes. As the name suggests, these enzymes remove one iodine atom from the T4 structure which changes its chemical structure to T3.
What happens when there is an imbalance in the thyroid gland?
The thyroid gland, due to a number of reasons, can become either overactive (hyperthyroidism) or underactive (hypothyroidism).
Overactive Thyroid (Hyperthyroidism)
An overactive thyroid means that the thyroid gland is producing excessive amounts of the thyroid hormone which then over-stimulates our metabolic functions. In this situation, you are said to be ‘hyperthyroid’. More on hyperthyroidism can be found here.
Underactive Thyroid (Hypothyroidism)
An underactive thyroid means that the thyroid gland is not producing enough thyroid hormone to perform its necessary metabolic functions. If we do not produce enough thyroid hormone then we are said to be ‘hypothyroid’. More on hypothyroidism can be found here.
Complications of thyroid disease
If left untreated there can be complications with both an overactive or underactive thyroid gland.
Possible complications with an overactive thyroid gland include:
- Eye problems, this can include a sensitivity to the light, watery eyes or double vision as well as damage to the surface of the eye itself
- Atrial fibrillation, a condition that causes abnormal heart rhythms
- Thinning of the heart muscle, which could lead to heart failure
- Osteoporosis (thinning of the bones), which then increases the risk of bone fractures
Possible complications with an underactive thyroid gland include:
- Elevated levels of cholesterol, which can increase the risk of cardiovascular disease, including heart attacks and stroke
- An abnormal swelling of the neck called ‘Goitre’
- In very rare cases an underactive thyroid can cause hypothyroid coma (myxoedema coma)
- Hypothyroidism can cause dementia if left untreated long-term
Both hypo- and hyperthyroidism are also associated with pregnancy complications.
It is important to identify and treat thyroid disease early in order to prevent these problems from happening.