Thyroid & Parathyroid

And related hormones.

Introduction

The hormones produced by the thyroid and parathyroid include the following.

Thyroxine (T4)
Triiodothyronine (T3)
Reverse Triiodothyronine (RT3)
Calcitonin
Parathyroid Hormone.

What are the thyroid and parathyroid glands?

The thyroid gland is a 2-inch-long butterfly shaped gland located at the base of the neck, in the front, and underneath the skin. It controls many important functions that occur in the body although its main job is to control the body’s metabolism, specifically the rate and speed by which the body transforms food consumed into useful energy. Considering that the cells in the body require energy to function, when the thyroid isn’t working right, it can impact the entire body.

The thyroid gland makes and secretes hormones that support the body’s metabolism. In addition, these hormones affect digestion, breathing, heart rate, blood pressure, brain development, body temperature, mental activity, skin and bone maintenance, and fertility.

The production of thyroid hormones is controlled by something called a ‘feedback loop’. The feedback loop includes the hypothalamus, pituitary gland, thyroid gland, and several hormones. To begin the feedback loop, the hypothalamus releases something called thyroid-releasing hormone (TRH), which stimulates the pituitary gland to release thyroid stimulating hormone (TSH). TSH is what triggers the thyroid to produce T4 and T3. The body also requires iodine for the release of T4 and T3 (which is one reason why salt is iodized, to prevent iodine deficiencies). The feedback loop closes once the levels of T4 and T3 increase, which tells the hypothalamus and pituitary to stop releasing TRH and TSH. Then when T4 and T3 levels drop the process begins again. Through this system, the body is able to maintain a consistent level of thyroid hormones in the body.

The parathyroid, on the other hand, consists of 4 small, pea-shaped glands located next to the thyroid. There are two parathyroid glands on each side of the neck. This gland produces something called parathyroid hormone which regulates the levels of calcium in the blood. It’s important to have calcium in the blood that is at a precise level because even small fluctuations in calcium may affect the proper functioning of both muscles and nerves.

What are the hormones produced and what do they do?

Thyroxine (T4): this is the primary hormone (approximately 80%) made and released by the thyroid gland, however it is relatively inactive. Once it is released, the body converts it into T3, which is the active thyroid hormone, meaning it has more of an effect on the body’s cells. The liver, muscles, kidneys, thyroid, pituitary, central nervous system, and brown fat tissue all have the ability to convert T4 to T3.
Triiodothyronine (T3): only 20% of the hormones released by the thyroid are T3, however T3 as the active thyroid hormone, has the majority of the effects on the body. The majority of T3 in the blood comes from its conversion from T4.
Reverse triiodothyronine (RT3): made in very small amounts by the thyroid gland, RT3 reverses the effects of T3.
Calcitonin: helps to regulate the amount of calcium in the blood stream
Parathyroid hormone (PTH): helps to control the amount of calcium in the blood. In addition, it helps to control the levels of phosphorus and vitamin D in the blood and bones. If the body detects low calcium levels in the blood, parathyroid hormone is released to regulate those levels. It can stimulate calcium to be released from the bones. It can signal the kidneys to maintain calcium rather than releasing it through urine. And it can signal the small intestine to absorb more calcium from foods. When blood calcium levels rise, the parathyroid will stop releasing PTH.

Diseases of the thyroid and parathyroid

Thyroid disease is actually quite common with an estimated 20 million people in the US alone living with the disease. Women, and people assigned female at birth, are about 5-8x more likely to get diagnosed with thyroid disease than men, or people assigned male at birth.

Thyroid disease can be split into two types: primary and secondary. In primary, the disease originates in the thyroid gland, e.g., a nodule exists on the thyroid that triggers the release of excess amounts of thyroid hormones. In secondary, the disease originates in the pituitary gland, e.g., a tumor in the pituitary triggers the release of excess amounts of TSH, which then stimulates the thyroid to produce excess T4 and T3.

The four main thyroid diseases include the following. Please note this is a high level overview and not an extensive list.

Hypothyroidism (underactive thyroid): happens when the body doesn’t produce and release enough thyroid hormones. This may cause aspects of metabolism to slow down. Causes of hypothyroidism may include Hashimoto’s (an autoimmune disease), thyroiditis (inflammation of the thyroid), deficiency of iodine, the thyroid simply isn’t functioning properly (usually seen from birth), over-treatment of hyperthyroid from medication use, and removal of the thyroid.
Hyperthyroidism (overactive thyroid): occurs when the thyroid produces and releases more hormones than the body needs. This may cause rates of metabolism to increase. Hyperthyroidism is less common than hypothyroidism. Causes may include Graves’ disease (an autoimmune condition), nodules on the thyroid, inflammation of the thyroid, inflammation of the thyroid after giving birth, an excess of iodine from diet or medication, over-treatment of hypothyroidism from medication, or benign tumors on the pituitary gland.
Goiter: an enlargement of the thyroid gland. These may be caused by the thyroid not making enough hormones, iodine deficiency, and certain medications.
Thyroid cancer: a very treatable cancer that originates in the thyroid gland. It is classified by the type of cells from which the cancer is produced, the majority being something called papillary.

Diseases of the parathyroid may cause parathyroid hormone levels to increase or decrease. An increase in parathyroid hormone may be caused by:

Parathyroid adenoma: a noncancerous growth on the parathyroid that causes it to produce excess hormone.
Genetic conditions: inherited conditions causing hyperparathyroidism.
Hyperplasia: when the parathyroid becomes enlarged and produces too much hormone.
Parathyroid cancer: a rare cancer that can cause it to produce too much hormone.

Likewise, diseases of the parathyroid that may cause it to produce too little parathyroid hormone include:

Damage: due to an accident or thyroid surgery.
Genetic conditions: a less common cause of too little PTH being produced.
Low levels of magnesium: the parathyroid needs magnesium in order to work properly, therefore low levels may cause it to produce less PTH than the body needs.
Autoimmune conditions: when the body’s immune system reacts negatively against its own tissues.

Symptoms of a misbehaving thyroid and parathyroid

Different conditions will have different symptoms and the most important thing is to know your body, recognize symptoms that might be new or different, and go get them checked out by your doctor.

Specific symptoms to look out for related to thyroid issues include the following:

Changes in your heartbeat, for example has your heart rate slowed down or sped up with no explanation.
New difficulties in being able to tolerate heat or cold.
Unexplained weight gain or weight loss
Irregular menses
Depression or anxiety

Specific symptoms to look out for related to high parathyroid hormone levels include the following:

More frequent urination and thirst
Joint or bone pain
Fatigue
Depression
Memory loss
Muscle aches, cramping, weakness
Nausea and vomiting
Constipation
A decrease in appetite

Specific symptoms to look out for related to low parathyroid hormone levels include the following:

Cramping in the muscles
Tingling in feet, fingers, or lips
Confusion or brain fog
Irregular heartbeat
Seizures
Dry brittle fingernails
Dry skin

Tests for thyroid and parathyroid hormone function

Luckily, if your provider suspects a malfunctioning thyroid or parathyroid, they may perform simple blood tests to confirm. These tests include the following:

TSH
Total T4
Free T4
Total T3
Free T3
Thyroid antibodies
Thyroglobulin
PTH

In conclusion

The hormones produced by the thyroid and parathyroid glands have important functions in the body. If you notice new or different symptoms make an appointment with your medical doctor as they may want to run a blood test that includes some of the above hormones.

References:

Cleveland Clinic. (2022). Hormones. Retrieved from https://my.clevelandclinic.org/health/articles/22464-hormones

Cleveland Clinic. (2021). Parathyroid Hormone. Retrieved from https://my.clevelandclinic.org/health/articles/22355-parathyroid-hormone

Cleveland Clinic. (2022). T3 (Triiodothyronine) Test. Retrieved from https://my.clevelandclinic.org/health/diagnostics/22425-triiodothyronine-t3

Cleveland Clinic. (2022). Thyroid. Retrieved from https://my.clevelandclinic.org/health/body/23188-thyroid

Cleveland Clinic. (). Thyroid Hormone. Retrieved from https://my.clevelandclinic.org/health/articles/22391-thyroid-hormone

Johns Hopkins Medicine. (n.d.). The Parathyroid Glands. Retrieved from https://www.hopkinsmedicine.org/health/conditions-and-diseases/the-parathyroid-glands

Johns Hopkins Medicine. (n.d.). Thyroid Disorders. Retrieved from https://www.hopkinsmedicine.org/health/conditions-and-diseases/disorders-of-the-thyroid

National Instittue of Health, National Library of Medicine, Medline Plus. (2020). Parathyroid Hormone (PTH) Test. Retrieved from https://medlineplus.gov/lab-tests/parathyroid-hormone-pth-test/

National Institute of Health, National Library of Medicine, Medline Plus. (2022). Thyroxine (T4) Tests. Retrieved from https://medlineplus.gov/lab-tests/thyroxine-t4-test/

National Institute of Health, National Library of Medicine, Medline Plus. (2021). Triiodothyronine (T3) Tests. Retrieved from https://medlineplus.gov/lab-tests/triiodothyronine-t3-tests/