WBR0274

Author	[[PageAuthor::Serge Korjian, M.D., Gonzalo A. Romero, M.D. [1] (Reviewed by Serge Korjian)]]
Exam Type	ExamType::USMLE Step 1
Main Category	MainCategory::Biochemistry, MainCategory::Pathophysiology, MainCategory::Physiology
Sub Category	SubCategory::Cardiology, SubCategory::Endocrine, SubCategory::Vascular, SubCategory::General Principles
Prompt	[[Prompt::A 56-year-old woman presents to the emergency department for 2 hours of continuous palpitations. The patient reports that for the past 6 months, she has noticed similar episodes that seemed to resolve spontaneously within a few minutes. She felt very anxious when her symptoms persisted and decided to seek care. The patient explains that she hasn't been herself lately; she has been constantly irritable and has been having trouble adapting to the hot weather. She has lost 4.5 kg (10 lbs) unintentionally and has noticed that her hair is becoming thinner. Which of the following is true about the hormone causing this patient’s syndrome?]]
Answer A	AnswerA::It decreases cardiac fatty acid metabolism
Answer A Explanation	AnswerAExp::By increasing heart rate and contractility and by stimulating β1 receptor expression, thyroid hormones cause an increase in fatty acid metabolism rather than a decrease.
Answer B	AnswerB::It decreases the number of cardiac β1 receptors
Answer B Explanation	AnswerBExp::Thyroid hormones stimulate the expression of a larger number of β1 receptors in the heart.
Answer C	[[AnswerC::It inhibits the Na+/K+ ATPase]]
Answer C Explanation	[[AnswerCExp::Thyroid hormones cause an increase in basic metabolic rate by primarily increasing the activity of the Na+/K+ ATPase. Inhibitors of the Na+/K+ ATPase include cardiac glycosides such as digoxin and ouabain.]]
Answer D	AnswerD::It increases the cardiac contractility
Answer D Explanation	AnswerDExp::Thyroid hormones increase cardiac function by increasing contractility, stroke volume, heart rate and subsequently cardiac output.
Answer E	AnswerE::It decreases stroke volume
Answer E Explanation	AnswerEExp::Thyroid hormones cause an increase in stroke volume given the increased contractility.
Right Answer	RightAnswer::D
Explanation	[[Explanation::Hyperthyroidism is caused by excessive production or release of T3/T4 from the thyroid gland most often from primary thyroid gland dysfunction and rarely from secondary, central disturbance. Classical symptoms of hyperthyroidism include anxiety, irritability, perspiration, palpitations, tachycardia and possible atrial arrhythmia, hypertension, heat intolerance, warm and moist skin, tremor, lid lag, weight loss, and menstrual abnormalities. Patients with Grave's disease (thyroid gland stimulating TSH receptor antibodies) may also have exophthalmus due to retro-orbital accumulation of glycosaminoglycans, periorbital edema, and pretibial myxedema (mucin accumulation in the dermis). Thyroid hormones have multiple functions that are executed by binding intra-nuclear receptors. The most important functions are related to an increase in basic metabolic rate including increase in Na+/K+ ATPase activity, glycogenolysis, gluconeogenesis, and lipolysis. T3 increases the expression of β1 receptors on cardiomyocytes. This leads to an increase in contractility, cardiac output, stroke volume, and heart rate. Educational Objective: Thyroid hormones cause an increase in basic metabolic rate including increase in Na+/K+ ATPase activity, glycogenolysis, gluconeogenesis, and lipolysis. T3 increases the expression of β1 receptors on cardiomyocytes. This leads to an increase in contractility, cardiac output, stroke volume, and heart rate. References: Klein I, Ojamaa K. Thyroid hormone and the cardiovascular system. N Engl J Med. 2001;344(7):501-9. Yen PM. Physiological and molecular basis of thyroid hormone action. Physiol Rev. 2001;81(3):1097-142.]]
Approved	Approved::Yes
Keyword	WBRKeyword::Thyroid, WBRKeyword::T3, WBRKeyword::T4, WBRKeyword::Hyperthyroidism, WBRKeyword::Cardiology
Linked Question	Linked::
Order in Linked Questions	LinkedOrder::