Euthyroid sick syndrome

Definitions & Key takeaways

Euthyroid sick syndrome (ESS) is a condition in which the thyroid gland is functioning properly, but the thyroid hormone levels are abnormally low. This can lead to a variety of symptoms, including fatigue, weight gain, and hair loss. Common causes of ESS include starvation or a serious illness, and is often seen in critically ill intensive care patients

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Introduction0:00–0:24

The term euthyroid sick syndrome, also known as nonthyroidal illness syndrome, can be broken down. Eu- refers to good and -thyroid refers to the thyroid gland which produces thyroid hormones.
So, euthyroid sick syndrome is a state where the thyroid gland is functioning normally, but the thyroid hormones are at abnormal levels.

Physiology0:24–2:23

Normally, the hypothalamus, which is located at the base of the brain, secretes thyrotropin-releasing hormone, known as ΤRH, into the hypophyseal portal system - which is a network of capillaries linking the hypothalamus to the anterior pituitary.
The anterior pituitary then releases a hormone of its own, called thyroid-stimulating hormone, thyrotropin or simply TSH.
TSH stimulates the thyroid gland which is a gland located in the neck that looks like two thumbs hooked together in the shape of a “V”.
If we zoom into the thyroid gland, we’ll find thousands of follicles, which are small hollow spheres whose walls are lined with follicular cells, and are separated by a small amount of connective tissue.
Follicular cells convert thyroglobulin, a protein found in follicles, into two iodine-containing hormones, triiodothyronine or T3, and thyroxine or T4.
Once released from the thyroid gland, these hormones enter the blood and bind to circulating plasma proteins. Only a small amount of T3 and T4 will travel unbound in the blood, and these two hormones get picked up by nearly every cell in the body.
Once inside the cell T­4 is mostly converted into T3, and it can exert its effect. T3 speeds up the basal metabolic rate.
So as an example, they might produce more proteins and burn up more energy in the form of sugars and fats. It’s as if the cells are in a bit of frenzy.
T3 increases cardiac output, stimulates bone resorption - thinning out the bones, and activates the sympathetic nervous system, the part of the nervous system responsible for our ‘fight-or-flight’ response.
Thyroid hormone is important - when you need a boost to get through the final rounds of a sporting competition or to stay warm during a snowstorm!
Meanwhile, thyroid hormones are involved in a number of other things, such as normal function of sebaceous and sweat glands, growth of hair follicles and regulation of the synthesis of proteins and mucopolysaccharides by skin fibroblasts.

Pathology2:23–3:55

In euthyroid sick syndrome, even though the thyroid gland is functioning normally, there is still a deficiency of thyroid hormones in the bloodstream, particularly T3.
The cause is not well understood, but it typically occurs in catabolic states - where the body is breaking down proteins like starvation or when there’s a serious illness.
When there’s not enough food available, our body adapts by conserving energy and one way to do that is to produce fewer structural proteins and enzymes.
One enzyme found in various body tissues including the thyroid, liver and kidneys is deiodinase which converts T­4 to the more potent T3.
In catabolic states, there’s less deiodinase and that leads to a lower level of T3 in the blood. Also during catabolic states, especially during a serious illness, immune cells produce cytokines like IL6, TNF alpha, and interferon-beta.
These cytokines act directly on the hypothalamus and pituitary gland to decrease the production of TSH and thyroid hormones respectively, which also leads to decreased T­4 and T3 levels.
Since the thyroid tissue is healthy and not directly affected in euthyroid sick syndrome, once the person starts eating again or once they recover from an illness, thyroid hormones levels return to normal.

Symptoms3:55–4:02

In euthyroid sick syndrome, there is usually no symptoms of hypothyroidism because it’s typically a temporary state. In diagnosing euthyroid sick syndrome, there's normal to low levels of T4, low T3, and normal or low levels of TSH, whereas in hypothyroidism TSH levels are usually elevated.

Diagnosis4:02–4:18

Treatment4:18–4:28

The treatment of euthyroid sick syndrome is based on treating the underlying cause. Once the cause has been treated, thyroid hormone levels return to normal.
All right, as a quick recap, euthyroid sick syndrome is where the thyroid tissue is not diseased, but the thyroid hormones are at unusually low levels.

Review4:28–4:47

It’s usually caused by starvation or a serious illness and can be distinguished from hypothyroidism by the normal or low levels of TSH.