ABIM - Thyroid Gland Pathophysiology

Here are key facts for ABIM Certification Exam 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. Hormone Regulation: Thyrotropin-releasing hormone (TRH) is released from the hypothalamus and carried in the pituitary circulation. In the anterior pituitary gland, TRH stimulates thyrotrophs (aka, tyrotropes) to release thyroid-stimulating hormone (TSH). TSH travels in the systemic circulation to its target organ, the thyroid, where it triggers release of the thyroid hormones, T3 and T4. 2. Feedback Mechanisms: Via negative feedback at the hypothalamus and the anterior pituitary gland, thyroid hormones inhibit further secretion of TRH and TSH. 3. T3/T4 Relationship: Thyroid hormones travel to the peripheral tissues, where some T4 is deiodinated to T3. Recall that T3 is more biologically active than T4. 4. Iodine Regulation: Wolff-Chaikoff effect: In cases of excessive iodine exposure, the thyroid inhibits iodine organification, thereby avoiding over-production of thyroid hormone. 1. Hyperthyroidism Labs: 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. Hypothyroidism Labs: Hypothyroidism is characterized by low levels of T3 and T4, hypercholesterolemia, and, in the case of primary hypothyroidism, high TSH. 3. Common Etiologies: Most common cause Hyperthyroidism: in the US is Graves disease" and "Hypothyroidism: Hashimoto thyroiditis. 1. Cardiovascular: Hyperthyroidism: tachycardia with palpitations. Patients experience dyspnea and chest pain and have systolic hypertension. vs. Hypothyroidism: bradycardia with reduced cardiac output. Patients experience dyspnea on exertion and are at risk for hypoventilation. 2. Neuropsychiatric: Hyperthyroidism: hyperactivity, restlessness, anxiety, and insomnia. Patient have increased reflexes and fine motor tremors. vs. Hypothyroidism has the opposite effects: patients are hypoactive, fatigued, and experience weakness and depressed mood with slow reflexes. 3. Metabolic: 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.

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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. 3. Normal thyroid function includes: Chronotropic and inotropic effects on the heart (heart rate and contractility). 1. Hypothyroid patients are at risk for hypoventilation. 2. Normal thyroid function includes: Maintain ventilatory responses to hypoxia and hypercapnia. 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 and accelerates bone growth and tissue turnover; thus, patients are at risk for osteoporosis and bone fractures. 2. Hypothyroidism also produces proximal weakness, but with myalgia; creatine kinase levels are often elevated. 3. Promote protein breakdown in muscle is a normal thyroid function. 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. 5. Normal thyroid functions include: Stimulate LDL receptors in the liver, Promote GI motility and carbohydrate absorption, and Promote lipolysis in adipose tissue. 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. 3. 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. 3. 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.

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Below is information not explicitly contained within the tutorial but important for ABIM. 1. Treatment strategies: Evidence-based approaches to hyperthyroidism and hypothyroidism management. 2. Subclinical disease: Current guidelines on when to treat subclinical thyroid dysfunction. 3. Radioactive iodine therapy: Patient selection, dose considerations, follow-up protocols. 1. Thyroid disease in pregnancy: Laboratory interpretation, treatment adjustments, fetal considerations. 2. Amiodarone-induced thyroid dysfunction: Mechanisms, diagnosis, management approach. 3. Critically ill patients: Interpretation of thyroid function tests, sick euthyroid syndrome. 1. Refractory hyperthyroidism: Treatment options when first-line therapies fail. 2. Thyroid hormone resistance syndromes: Diagnosis and management. 3. Thyroid dysfunction with comorbidities: Management modifications for patients with cardiac disease, renal disease, diabetes.