The thyroid is a large ductless gland in front of the neck, which secretes hormones regulating growth and development of the body by the process of metabolism. It does this by secreting its own hormones in the blood.

A thyroid disease or the thyroid disorder causes a disturbance in the thyroid function resulting in abnormal thyroid secretion levels of the thyroid hormones, T3 and T4.

This gives rise to symptoms of thyroid disease such as weight loss or weight gain and some such symptoms, which are explained in the subsequent posts related to the thyroid disease.

You may have an overactive thyroid, which over functions or an underactive thyroid gland that does not function well.

Thyroid Gland Anatomy

Just as we have a circulatory system, a skeletal system, a nervous system, we also have in our body, the endocrine system.

The word “endocrine” comes from two Greek words: endo and krinein, which mean  “secreting internally”.

And therefore, as the meaning suggests, the endocrine system consists of glands, which secrete hormones internally into the blood and not through ducts into the bloodstream. Such glands include the pituitary gland, the adrenal gland, the thyroid gland.

The normal thyroid gland has a butterfly shape and it wraps itself around the trachea (windpipe) in front of the lower neck, just above the collarbone.

It extends from the fifth cervical vertebra (above) down to the first thoracic (below).

It has two lobes on either side of the trachea, joined by a narrow band of thyroid tissue called the isthmus.

It is a vascular structure and brownish red in color. The nerves supplying the larynx (voice box) pass through it. Its function and hormone activity are described below.

In normal circumstances, you cannot feel the thyroid with your fingers. What you feel is Adam’s apple, which is the name given to the thyroid cartilage.

Thyroid hormones and their normal levels

The thyroid gland produces thyroxine (T4 because it contains four iodine atoms), which is basically a prohormone (a relatively inactive intraglandular precursor of a hormone) and lesser amounts of the active hormone, triiodothyronine (T3).  Collectively, thyroxine and triiodothyronine are referred to as the thyroid hormones.The thyroid gland releases about 20 times more T4 than T3.

Twenty percent of the body’s T3 is made by the thyroid gland; the other 80% comes from thyroxine converted by the body organs such as the liver or kidneys.

Calcitonin is another hormone, which the thyroid gland produces from its C-cells.

The normal range of thyroid hormones

  • The normal range of TSH levels is 0.4 to 4.0 milli-international units per liter. Since 2003, this normal range has been revised to 0.3 to 3.3. However, some labs do still use the old normal range.
  • Normal T3 range: Total T3 is 80 to 220 points. Free T3 is 2.3 to 4.2 points.
  • Normal T4 Level: Total T4 is 4.5 to 12.5. Free T4 is 0.7 to 2.0

Thyroid Function

  • The thyroid performs important functions. For it to function well, you must have a diet that contains a good amount of iodine. The thyroid cells are the only cells in the body that take up iodine.
  • The Thyroid gland takes up the iodine and amino acid called Tyrosine from the food we eat and forms thyroid hormones called T3 (Thyroxine) and T4 (Triiodothyronine). These hormones are important for of the development of the brain during infancy and childhood.
  • T3 is the more active hormone and controls most of the metabolic activity. T4 is ultimately converted into T3 by removal of an iodine atom. This occurs mainly in the liver and other tissues such as the brain where T3 is active.
  • The thyroid releases these hormones into the blood when commanded by the pituitary gland situated in the brain.
  • T3 and T4 control the metabolism in the body cells, maintaining the rate at which the body utilizes fats and carbohydrates and ultimately the conversion of calories and oxygen into energy.
  • The metabolism in our body is totally dependent on the thyroid gland.
  • Other functions include influencing the heart rate and digestive function, regulating the production of proteins, muscle control, and bone maintenance.
  • Calcitonin also secreted by the thyroid regulates the calcium and phosphate levels in the blood.

What controls the thyroid gland?

The functioning of the thyroid gland is controlled by a small peanut-sized gland, situated at the base of the brain, called the pituitary gland. It secretes the hormone called Thyroid stimulating hormone (TSH).

When the levels of T3 and T4 in the blood fall and the requirement for metabolism arises, the pituitary gland secretes TSH (thyroid stimulating hormone) into the blood, thereby stimulating the thyroid to release T3 and T4 into the blood.

When high levels of T3 and T4 are not required, the pituitary stops the secretion of the TSH and the thyroid stops releasing T3 and T4 into the blood.

The pituitary gland is in turn controlled by another gland in the brain called the Hypothalamus, which secretes the Thyrotropin regulating hormone (TRH).

Thyroid Diseases or Disorders

Hyperthyroidism results due to an overactive thyroid gland, which leads to increased levels of thyroid hormones in the blood. It is often caused by Graves disease or a thyroid nodule.

Hypothyroidism is an underactive functioning thyroid gland. It results in low production and low levels of thyroid hormones in the blood. Autoimmune disease is the most common cause.

Goiter is enlargement of the thyroid gland. At times, it presses on the neighboring structures such as the trachea or the esophagus and can cause difficulty in breathing and swallowing. Treatment is surgical removal.

Malignancy of the thyroid gland begins in a thyroid nodule. Malignancy is found in 7% of the people with the thyroid nodule, but life expectancy is quite long. It is treated with surgery, radiation, and hormonal treatments.

Inflammation of the Thyroid (Thyroiditis) is an inflammation of the thyroid and causes pain and fever. It may manifest as symptoms of hyper or hypothyroidism and treated accordingly.

Thyroid Nodule: The thyroid may develop a nodular growth. It is usually benign and present in more than 40% of the population.

Thyroid storm: It is a form of hyperthyroidism that is rare. In this disorder, the thyroid hormones rise very high and can cause serious diseases, which can be life-threatening.