The Anatomy of the Thyroid Gland

The thyroid gland is part of the endocrine system (along with the adrenal glands, hypothalamus, pituitary, ovaries, and testes). The thyroid gland releases hormones into the bloodstream to control your metabolism, which is the primary way your body uses energy. In addition to metabolism, the hormones it releases also help with processes like bone growth, brain development, heart rate, digestion, muscle functioning, body temperature, menstrual cycles, and more. The thyroid can also produce more hormones when needed, such as to help increase body temperature or when a woman is pregnant. Hashimoto’s disease and Graves’ disease are causes of abnormal hormone production.

Anatomy

The thyroid gland is located in the front of the neck, right below the larynx and next to and around to the trachea. It’s a butterfly shape due to the gland consisting of two lobes that are connected by a piece of tissue called the isthmus. Each lobe is filled with follicles that contain hormones the body needs to function. Two capsules surround the thyroid gland—an outer layer that connects to the voice box muscles and surrounding nerves, and one in between this layer and the thyroid gland that allows the thyroid to move when swallowing or talking.

There are also two types of cells that make up thyroid tissue—follicular cells and parafollicular cells. These two cells are responsible for producing certain hormones that the thyroid gland then secretes into the bloodstream. Follicular cells (also referred to as thyroid epithelial cells, which makes up a majority of the thyroid gland) create thyroxine (T4) and triiodothyronine (T3), which are the major metabolism-regulating hormones, while the parafollicular cells (also called C cells) create calcitonin, which helps regulate calcium and phosphate levels in the blood.

Anatomical Variations

There are a number of variations the thyroid gland can take on, and this may affect how the thyroid operates and what disorders arise because of these differences. In a study of 52 male cadavers and 18 females, 9.6% of the males and 5.6% of females were missing the isthmus in their thyroid gland.

It’s also possible for lobes of the thyroid to be different sizes from one another. Some individuals have a pyramidal lobe, which is considered a third lobe in the thyroid that stems out from the isthmus. Some thyroid glands may or may not also have levator glandulae thyroideae, a fibrous band that stretches from a pyramidal lobe to the isthmus.

In certain cases, the thyroid gland can become enlarged (known as a condition called goiter) or develop clumps of cells called thyroid nodules, which are often benign but can sometimes indicate thyroid cancer.

Function

The thyroid gland is controlled by the hypothalamus and the pituitary gland, which are both located in the brain. The hypothalamus releases thyrotropin-releasing hormone (TRH), which then tells the pituitary gland to release thyroid-stimulating hormone (TSH). Together the hypothalamus and pituitary gland know when thyroid hormone levels are too high or too low, and by secreting an appropriate amount of TRH and TSH they can signal to the thyroid gland how much or how little hormones it needs to make.

One of the most important elements behind producing thyroid hormones is iodine, which we get a majority of through food or supplements. Both T3 and T4 need iodine in order to be produced by the thyroid gland. Once iodine makes its way to the thyroid it gets converted into T3 and T4. These are then released into the bloodstream to help with multiple functions like increasing the metabolic rate in the body, growth, brain development, and more. Some of the highest dietary sources of iodine include cheese, cow’s milk, eggs, saltwater fish, soy milk, and yogurt.

Associated Conditions

Depending on whether or not a thyroid gland is overactive or not producing enough hormones, some disorders can stem from this. Common thyroid diseases include:

• Hyperthyroidism
• Hypothyroidism
• Hashimoto’s disease
• Graves’ disease
• Goiter
• Thyroid nodules
• Thyroid cancer

Tests

Your healthcare provider can conduct a series of blood tests to determine if you may have a thyroid disorder, in addition to seeing how well your thyroid gland is functioning. These include:

• TSH test: The pituitary gland produces TSH, and this tells the thyroid gland the amount of hormone it needs to make. If you have a high TSH level, it means you may have hypothyroidism. Your thyroid isn’t making enough hormones, so the pituitary gland keeps releasing TSH to try to signal it to kick up its hormone production. Alternatively, low TSH levels can signal hyperthyroidism, as the thyroid hormone is producing too many hormones and the pituitary gland is trying to get it to slow down by halting the release of TSH.
• Total thyroxine (T4) test: In certain cases, T4 levels may be higher or lower not due to a thyroid disorder (such as when you’re pregnant or if you’re taking certain medications). But if a pre-existing condition isn’t behind your T4 levels, high T4 may indicate hyperthyroidism while low T4 may be caused the hypothyroidism.
• Triiodothyronine (T3) test: If your T4 levels are normal, but you’re still exhibiting symptoms of a thyroid disorder T3 levels will be tested at the same time. Similar to the T4 test, high or low T3 levels can indicate either hyperthyroidism or hypothyroidism.
• Thyroid antibody test: The level of antibodies in the blood can help pinpoint if your thyroid disorder is due to an autoimmune condition like Hashimoto’s disease or Graves’ disease. Thyroid antibodies like TPO or TRAbs are antibodies that the body makes which target thyroid tissue leading to gland destruction or abnormal function.

In addition to these blood tests, an ultrasound, thyroid scan, or radioactive iodine uptake test can be done to check thyroid function and find the exact cause behind a hyperthyroidism or hypothyroidism diagnosis as well as examining any nodules or abnormalities on the thyroid gland. Getting bloodwork is always the first step and will help your healthcare provider decide if further testing is needed.