Euthyroid sick syndrome

A 67-year-old man comes to the PA complaining of decreased concentration for the last 5 months. He is no longer able to complete his daily crossword puzzles due to lack of focus and a persistent feeling of fatigue. His partner states that he has been more forgetful recently, oftentimes misplacing common objects such as his glasses and wallet. The patient has gained 12-lb in the last 4 months without a change in diet or activity level. Past medical history is significant for hypertension, which is being treated with lisinopril. The patient has had no changes in vision, headaches, or loss of peripheral sensation. Temperature is 37.0°C (98.6°F), pulse is 50/min, blood pressure is 128/86 mm Hg, and BMI is 27 kg/m². Physical examination shows dry skin and a slightly distended abdomen. Deep tendon reflexes are delayed in the upper and lower extremities. Laboratory studies show the following:

Laboratory value	Result	Reference Range
Sodium	137 mEq/L	136-146 mEq/L
Potassium	4.8 mEq/L	3.5-5 mEq/L
Chloride	99 mEq/L	95-105 mEq/L
Creatinine	0.6 mg/dL	0.6-1.2 mg/dL
Glucose	89 mg/L	70-110 mg/dL
Hemoglobin	12.6 g/dL	13.5-17.5 g/dL
Hematocrit	42%	41-53%

Which of the following is the most likely cause of this patient’s condition?

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.

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 - and the occasional increase can be really useful 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.

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

Sources

"Robbins Basic Pathology" Elsevier (2017)

"Harrison's Principles of Internal Medicine, Twentieth Edition (Vol.1 & Vol.2)" McGraw-Hill Education / Medical (2018)

"Pathophysiology of Disease: An Introduction to Clinical Medicine 8E" McGraw-Hill Education / Medical (2018)

"CURRENT Medical Diagnosis and Treatment 2020" McGraw-Hill Education / Medical (2019)

"Harrison's Endocrinology, 4E" McGraw-Hill Education / Medical (2016)

"Mechanisms behind the non-thyroidal illness syndrome: an update" Journal of Endocrinology (2009)

"Homeostatic Control of the Thyroid–Pituitary Axis: Perspectives for Diagnosis and Treatment" Frontiers in Endocrinology (2015)

Euthyroid sick syndrome

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