USMLE/COMLEX 3 - Thyroid Gland Pathophysiology

Here are key facts for USMLE Step 3 & COMLEX-USA Level 3 from the Thyroid Gland Pathophysiology tutorial, as well as points of interest at the end of this document that are not directly addressed in this tutorial but should help you prepare for the boards. See the tutorial notes for further details and relevant links.

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1. Lab interpretation: Hyperthyroidism is characterized by elevated levels of T3 and T4, low LDL (due to increased LDL receptors in the liver), and, if in the case of primary hyperthyroidism, low levels of TSH due to negative feedback on the pituitary. 2. Lab interpretation: Hypothyroidism is characterized by low levels of T3 and T4, hypercholesterolemia, and, in the case of primary hypothyroidism, high TSH. 3. Regulatory pathway: Via negative feedback at the hypothalamus and the anterior pituitary gland, thyroid hormones inhibit further secretion of TRH and TSH. 4. Etiologic considerations: In some countries, lack of iodine is the primary cause of goiter, but many countries, including the U.S., add iodine to salt and other foods so that deficiency is rare. 5. Epidemiology: Most common cause Hyperthyroidism: in the US is Graves disease and Hypothyroidism: Hashimoto thyroiditis. 1. Cardiovascular presentations: Hyperthyroidism: tachycardia with palpitations. Patients experience dyspnea and chest pain and have systolic hypertension. vs. Hypothyroidism: bradycardia with reduced cardiac output. 2. Neurological presentations: Hyperthyroidism: hyperactivity, restlessness, anxiety, and insomnia. Patient have increased reflexes and fine motor tremors. vs. Hypothyroidism: hypoactive, fatigued, and experience weakness and depressed mood with slow reflexes. 3. Metabolic derangements: Hyperthyroidism: heat intolerance with increased sweating; patients have elevated basal metabolic rate with weight loss. vs. Hypothyroidism: cold intolerance and reduced sweating; patients have lower basal metabolic rates with weight gain, and possibly hyponatremia. 4. Integumentary clues: Hyperthyroidism: vasodilation produces warm, moist skin vs. Hypothyroidism: patients have reduced blood flow with cool, dry skin. Hair is often coarse and brittle, and patients may have alopecia.

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1. "Hyperthyroidism: tachycardia with palpitations. Patients experience dyspnea and chest pain and have systolic hypertension." 2. "Hypothyroidism: bradycardia with reduced cardiac output. Patients experience dyspnea on exertion and are at risk for hypoventilation." 1. Hyperthyroidism: hyperactivity, restlessness, anxiety, and insomnia. Patient have increased reflexes and fine motor tremors. 2. Hypothyroidism has the opposite effects: patients are hypoactive, fatigued, and experience weakness and depressed mood with slow reflexes. 1. Hyperthyroidism: breaks down proteins and muscles, producing proximal weakness with normal levels of creatine kinase; hypothyroidism also produces proximal weakness, but with myalgia; creatine kinase levels are often elevated. 2. Hyperthyroidism: accelerates bone growth and tissue turnover; thus, patients are at risk for osteoporosis and bone fractures. Hypothyroidism in children produces impaired bone formation and growth. 1. Hyperthyroidism: more bowel movements, possibly with diarrhea, and appetite is increased. 2. Hypothyroidism: constipation and reduced appetite. 3. Hyperthyroidism: heat intolerance with increased sweating; patients have elevated basal metabolic rate with weight loss. 4. Hypothyroidism: cold intolerance and reduced sweating; patients have lower basal metabolic rates with weight gain, and possibly hyponatremia. 1. Hyperthyroidism: vasodilation produces warm, moist skin; hair is often fine (thin diameter). 2. Hypothyroidism: patients have reduced blood flow with cool, dry skin. Hair is often coarse and brittle, and patients may have alopecia. Nails are brittle. Hypothyroidism is often characterized by nonpitting edema and puffy facies, due to water retention – thus its alternative name, myxedema. 1. Hyperthyroidism: irregular or reduced menstrual flow, gynecomastia, lower libido, and possibly reduced fertility. 2. Hypothyroidism: similar effects, but gynecomastia is not common. 1. Both hyper- and hypothyroidism: can produce periorbital edema. 2. Hyperthyroidism is also associated with exophthalmos, aka, proptosis, which is characterized by 'bulging' eyeballs due to inflammation around the eyes. Lid retraction can also occur in hyperthyroidism due to increased sympathetic stimulation of the muscles of the eyelid. 1. Sympathetic nervous system: thyroid hormone increases beta adrenergic receptor numbers and sensitivity. This can help us predict the physiological roles of thyroid hormone and the pathology that results when there is too much or too little hormone activity. 2. Wolff-Chaikoff effect: In cases of excessive iodine exposure, the thyroid inhibits iodine organification, thereby avoiding over-production of thyroid hormone.

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Below is information not explicitly contained within the tutorial but important for USMLE & COMLEX 3. 1. Medication selection and monitoring: Age and comorbidity considerations in thyroid replacement therapy. 2. Subclinical thyroid disease: Management approach to abnormal labs without symptoms. 3. Perioperative management: Considerations for patients with thyroid disorders undergoing surgery. 1. Pregnancy: Management of thyroid disorders during pregnancy and postpartum. 2. Elderly: Modified treatment goals and medication dosing. 3. Critical illness: Management of sick euthyroid syndrome. 1. Cardiovascular complications: Management of atrial fibrillation in thyroid disorders. 2. Neuropsychiatric sequelae: Addressing cognitive impairment and mood disorders. 3. Thyroid disease in multi-system disorders: Approach to autoimmune polyendocrine syndromes.