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Endocrine system




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USMLE® Step 1 questions

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A 63-year-old man comes to the clinic for the first time complaining of excessive fatigue. He moved three months ago and is looking to establish care with a new physician. The patient states that he has felt tired and lethargic for the last 2 weeks. The patient is taking a medication for a “heart condition,” but he is unable to recall the name. He has also been taking acetaminophen daily to help relieve pain in his sore arms, which also started several weeks ago, and laxatives to help with bowel movements, which have slowed recently. Temperature is 37.2°C (99.0°F), pulse is 58/min and irregular, and blood pressure is 128/82 mm Hg. Physical examination reveals a slightly bluish hue to his skin. The upper extremities are diffusely tender to palpation. Laboratory studies are as follows:  
 Laboratory value  Result 
 Sodium  137 mEq/L 
 Potassium  4.8 mEq/L 
 Chloride  97 mEq/L 
 Glucose  97 mEq/L 
 Creatine kinase (CK)  145 U/L 
 Aspartate aminotransferase  66 U/L 
 Alanine aminotransferase  77 U/L 

Which of the following medications was this patient most likely taking?

External References

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Amiodarone p. 329

hypothyroidism p. 250

hypothyroidism with p. 347

Autoimmune hypothyroidism p. 171

Fetal hypothyroidism p. 347


maternal hypothyroidism from p. 347

Hypothyroidism p. 346, 347

amiodarone and p. 329

anemia p. 428

in anemia taxonomy p. 425

carpal tunnel syndrome and p. 456

as drug reaction p. 250

hormone replacement p. 362

lithium p. 598

Lithium p. 598

hypothyroidism p. 347

Pregnancy p. 657

hypothyroidism in p. 348

Sulfonamides p. 191

hypothyroidism p. 250


In hypothyroidism, ‘hypo' refers to having too little, and ‘thyroid’ refers to thyroid hormone, so hypothyroidism refers to a condition where there’s a lack of thyroid hormones.

Normally, the hypothalamus, which is located at the base of the brain, detects low blood levels of thyroid hormones and releases thyrotropin-releasing hormone into the hypophyseal portal system - which is a network of capillaries linking the hypothalamus to the anterior pituitary.

The anterior pituitary then releases thyroid-stimulating hormone, also called 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”.

The thyroid gland is made up of thousands of follicles, which are small spheres lined with follicular cells. 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 cell’s basal metabolic rate.

So as an example, the cell 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.


Hypothyroidism refers to a condition where there's a lack of thyroid hormones (triiodothyronine (T3) and thyroxine (T4)), which normally help control the body's metabolism.

Hypothyroidism can be primary or secondary. In primary hypothyroidism, the thyroid gland isn't working properly, because of an autoimmune disease like Hashimoto's thyroiditis, hyperthyroidism treatment, or a congenital defect. In secondary hypothyroidism, either the anterior pituitary gland or the hypothalamus is the problem, usually because of a tumor or damage from surgery.

Symptoms of Hypothyroidism include weight gain, mental slowness, swelling in the skin and soft tissues, and a slower heart rate.


  1. "Robbins Basic Pathology" Elsevier (2017)
  2. "Harrison's Principles of Internal Medicine, Twentieth Edition (Vol.1 & Vol.2)" McGraw-Hill Education / Medical (2018)
  3. "Pathophysiology of Disease: An Introduction to Clinical Medicine 8E" McGraw-Hill Education / Medical (2018)
  4. "CURRENT Medical Diagnosis and Treatment 2020" McGraw-Hill Education / Medical (2019)
  5. "Harrison's Endocrinology, 4E" McGraw-Hill Education / Medical (2016)
  6. "Hypothyroidism" The Lancet (2017)
  7. "Hypothyroidism in Context: Where We’ve Been and Where We’re Going" Advances in Therapy (2019)
  8. "Hypothyroidism" The Lancet (2017)

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