All hormones in our body must work in concert to maintain optimal health. Thyroid gland hormones play a major role in regulating this work. In today’s article, we will discuss which symptoms can tell us that not everything is in order with our thyroid hormones. We will also take a closer look at the three ways to check the functioning of the thyroid gland.

This article was last reviewed by Svetlana Baloban, Healsens, on January 24, 2020. This article was last modified on 17 February 2020.

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Heart

Thyroid gland hormones have a permissive effect on catecholamines. Catecholamines are a group of similar hormones produced by the adrenal medulla. Those, in turn, increase the expression of beta-receptors to increase heart rate, stroke volume, cardiac output, and contractility⁴. Often, people with hypothyroidism have very high cholesterol levels (350 and higher), and a bunch of other risk factors leading to a cardiovascular disease⁵.

Thermoregulation

Thyroid hormones are responsible for thermoregulation, which is why people with reduced thyroid function (hypothyroidism) often complain that they are constantly cold. This happens due to increased available energy in the body, as well as increased appetite, heart rate, and the amount of oxygen delivered to various parts of the body.

Metabolism

These hormones also help control the basal metabolic rate (the rate at which calories are burned). So fatigue and excess weight are also signs of hypothyroidism. On average, an underactive thyroid gland can add 5 to 15 kg of weight. But most of that extra weight will come from water and salt. In addition, decreased thyroid function can cause constipation⁶, memory problems, and lethargy.

An overactive thyroid gland (hyperthyroidism) is characterized by the opposite signs: having difficulty gaining weight, feeling hot, having frequent bowel movements, feeling nervous, and having a rapid heart rate. Other functions controlled by thyroid hormones include metabolism of fats, proteins and carbohydrates, as well as protein synthesis.

Relation to other hormones

There is also an interrealtion between thyroid function and control over other hormone levels. So, deviation of the thyroid hormone level from the norm can adversely affect the work of the adrenal glands. For instance, сertain human autoimmune conditions can destroy both the thyroid gland and adrenal cortex resulting in combined hormone deficiencies⁷. Moreover, dysfunction of the thyroid gland negatively affects the levels of sex hormones. Hyper- and hypothyroid men have increased rates of sexual dysfunction, including erectile dysfunction (ED). But the good news is that treatment of thyroid disorder at least partially reverses sexual dysfunction⁸. Thyroid hormone also affects fertility, ovulation, and menstruation⁹.

What hormones does the thyroid gland produce? In the following, we will discuss three tests for testing thyroid function – thyroid stimulating hormone (TSH), free T3, and free T4. You will probably want to make these tests part of your early diagnosis program and get them done regularly.

Thyroid stimulating hormone (TSH)

Thyroid stimulating hormone is a hormone that stimulates the thyroid gland. It is also known as TSH and is a glycoprotein hormone produced by the anterior pituitary gland. It is the main stimulus for the production of thyroid hormones. The TSH screening test is the first line test for suspected thyroid disease.

According to the American Thyroid Association, TSH levels are typically between 0.4 and 4.0 milliunits per liter (mU / L). However, these ranges will vary among laboratories and the upper limit can be raised from 4 to 5. If your level is higher than this, you are most likely to have an underactive thyroid gland. However, some studies show that this range should actually lie within 0.45–2.5 mIU / L. This is because TSH below the upper limit of the normal reference range poses an additional risk of progression to overt hypothyroidism over time¹⁰¹¹.

So, high TSH levels indicate that your thyroid gland is not producing enough thyroid hormones. This condition is called hypothyroidism, which we wrote about above. Low TSH levels, on the other hand, may mean that your thyroid is producing too much hormone, the condition is called hyperthyroidism. Hyperthyroidism can be caused by Graves’ disease. This is a condition when your body’s immune system attacks the thyroid gland. It can also be a result of too much iodine in your body.

T3 (triiodothyronine) and T4 (thyroxine)

T3 (triiodothyronine) and T4 (thyroxine) are the two key thyroid hormones that are produced by the thyroid gland. The main product of thyroid secretion is T4, while T3 is secreted only in small amounts¹². However, the activity of T3 is three to four times higher than that of T4. Nonetheless, both hormones play a vital role. Both T3 and T4 contain iodine. And it is iodine that is one of the main building blocks of both hormones. So let’s talk about tests that help measure the content of these hormones in the body.

Triiodothyronine (T3)

So, T3 thyroid hormone is the most active of the two main hormones. Normal levels of total T3 (free and bound) in the blood are between 75 and 195 ng / dL. The normal level of free T3 in the blood is 0.2-0.5 ng / dL.

If your results show high levels of total T3 or free T3, this could mean that you have hyperthyroidism. High T3 levels can lead to unpleasant symptoms such as rapid heart beat, insomnia and anxiety. High T3 levels also can harm the heart and the bones.

Low T3 levels may mean you have hypothyroidism. Typically, the T3 test results are often compared to the T4 and TSH test results. This helps to more accurately diagnose thyroid disease and understand whether the disease is primary or secondary. Primary hypothyroidism is most common; it is associated with thyroid disease and high thyroid-stimulating hormone (TSH) levels. Secondary hypothyroidism is less common; it is associated with a disease of the pituitary gland or hypothalamus. The TSH level will be low.

So, if we talk about the role of triiodothyronine, then it regulates the rate of oxygen consumption by tissues, and also stimulates protein synthesis, gluconeogenesis and glycogenolysis (which leads to an increase in the concentration of glucose in the blood). It is also responsible for lipolysis and intestinal motor function. In addition, triiodothyronine enhances catabolism and excretion of cholesterol with bile, promotes the synthesis of vitamin A and the absorption of vitamin B12 in the intestine, bone growth, and the production of sex hormones.

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Thyroxine (T4)

A normal total T4 level in adults ranges from 5.0 to 12.0μg/dL. As for free T4, the normal range in adults is 0.8 to 1.8 nanograms per deciliter (ng / dL) or 12 to 30 pmol / L. In general, high thyroxine T4 levels (as well as total T4, free T4, or free T4 index) may indicate an overactive thyroid gland (hyperthyroidism). A low level may indicate an underactive thyroid gland (hypothyroidism).

Just like in situation with T3, doctors don’t only look at your T4 level, but also at your TSH level to diagnose hypothyroidism. So, if TSH> 4.0 / mU/L, and the T4 level is low, then this is hypothyroidism. However, if your TSH is higher than  4.0 mU / L and your T4 level is normal, then your doctor will most likely suggest testing your serum thyroid peroxidase antibodies (anti-TPO). If these antibodies are present, it may indicate an autoimmune thyroid disorder, which is a risk factor for hypothyroidism¹³. In this case you will be advised to take a TSH test at least once a year.

Higher T4 or lower TSH levels are known to be associated with an increased risk of dementia¹⁴.

In addition to the two hormones described above, there is another hormone produced by the thyroid gland. It is called calcitonin. Calcitonin is produced by C cells and is involved in calcium and bone metabolism.

The role of the pituitary gland

In conclusion, we’d like to say a few words on how the pituitary gland affects the production of thyroid hormones. Sometimes our body needs more thyroid hormones, and sometimes fewer. For example, when we grow up, feel cold or are pregnant, the thyroid gland, to produce the right amount of hormones, needs help of another gland: the pituitary gland. The pituitary gland tells the thyroid gland whether it needs to release more hormones or fewer hormones into the bloodstream. Also, a certain amount of thyroid hormones are needed to transport proteins in the blood. If the body needs more hormones, T3 and T4 can be released from blood proteins and do their job.

👉Thus, there is no uniform recommendation on the need for screening for thyroid hormones!¹⁵

☝️For example, the American Thyroid Association recommends TSH screening for all adults over 35, with retests every 5 years. However, AACE recommends routine screening for TSH in elderly patients regardless of their age. But the USPSTF still discourages routine thyroid screening in non-pregnant adults.

FURTHER READING

1. Haeger K. London: Harold Starke; 1988. The illustrated history of surgery
2. General Information/Press Room
3. General Information/Press Room
4. Physiology, Thyroid Hormone
5. Effects of Thyroid Dysfunction on Lipid Profile
6. Gastrointestinal manifestations of endocrine disease
7. A Direct Role for Thyroid Hormone in Development of the Adrenal Cortex
8. Effect of Thyroid Hormone Derangements on Sexual Function in Men and Women
9. Physiology, Thyroid Hormone
10. The evidence for a narrower thyrotropin reference range is compelling
11. Thyrotropin and thyroid antibodies as predictors of hypothyroidism: a 13-year, longitudinal study of a community-based cohort using current immunoassay techniques
12. THYROID FUNCTION
13. How Doctors Diagnose Hypothyroidism
14. Higher FT4 or TSH below the normal range are associated with increased risk of dementia
15. New Guidelines for Thyroid Disease Screening: Perspectives for Pharmacists