38 Questions
Which of the following inhibits the iodination of tyrosine residues of thyroglobulin?
Thioamides
What is the result of the coupling of DIT and MIT?
T3
Where are T4 and T3 released from?
Thyroglobulin
What is the result of the coupling of DIT and DIT?
T4
Which of the following drains into the brachiocephalic vein?
Inferior thyroid vein
What is the term for the conversion of T4 to T3 in peripheral tissue?
Deiodination
In iodine-sufficient areas, what is the most common cause of hypothyroidism?
Chronic autoimmune (Hashimoto) thyroiditis
What is the path of the left recurrent laryngeal nerve?
Posteriorly around the aortic arch to ascend through the superior mediastinum
What is the effect of increased TSH levels on the thyroid gland?
Increased gland size and vascularity
What is the most sensitive test for hypothyroidism?
TSH levels
What is a common musculoskeletal symptom of hypothyroidism?
Arthralgia
What is a common pulmonary symptom of hypothyroidism?
Dyspnea on exertion
What is a common genitourinary symptom of hypothyroidism?
Menstrual irregularity
What is characteristic of subclinical hypothyroidism?
Elevated TSH and normal free T4
What percentage of thyroid hormone is synthesized by the thyroid gland?
90%
What is the boundary of the thyroid gland anteriorly?
Carotid sheath
What is the most common cause of hypothyroidism worldwide?
Iodine deficiency
What is the function of thyroid peroxidase antibodies?
Catalyzing the oxidation of iodine
What is the classification of hypothyroidism in order of decreasing frequency?
Thyroid, pituitary, hypothalamic failure
What is the percentage of thyroid peroxidase antibodies elevated in thyroid disease?
11.3%
What is the type of laryngeal nerve that is not active?
Superior laryngeal nerve
What is the percentage of free thyroid hormone in the bloodstream?
0.04%
What is the definition of overt thyrotoxicosis?
High serum T4 and T3 concentrations and low serum TSH concentrations
What is the most common cause of thyrotoxicosis?
Grave's disease
What is the term for a late manifestation of hypothyroidism characterized by coma or precoma?
Myxedema coma
What is the characteristic of patients with subclinical thyrotoxicosis?
Normal serum T4 and T3 concentrations and low serum TSH concentrations
What is the common complication of overtreatment with an excessive dose of T4 in postmenopausal women?
Bone loss
What is the term for the pathophysiological mechanism underlying thyrotoxicosis?
Unregulated release of T4 and T3
What is the characteristic of patients with myxedema coma?
Extreme hypothermia
What is the percentage of patients with thyrotoxicosis who have Grave's disease?
60%-85%
What is the term for the transient or persistent syndrome that results from exposure to excessive concentrations of thyroid hormones?
Thyrotoxicosis
What is the characteristic of patients with overt thyrotoxicosis?
Most patients have symptoms and signs of this entity
What is the typical characteristic of the increase in T3 and T4 production in patients with thyrotoxicosis?
The increase in T3 production is greater than the increase in T4 production
Which of the following groups of people is most likely to have high serum T3 concentrations but normal serum T4 concentrations?
Some patients with thyrotoxicosis
What is the most common cause of thyrotoxicosis?
Graves' disease
In which of the following groups is Graves' disease most likely to occur?
Women 30 to 60 years old
Which of the following hormones may stimulate the production of thyroid-stimulating hormones?
Chorionic gonadotropin
In which of the following situations is it particularly difficult to maintain a euthyroid state?
In patients in whom it is difficult to render and maintain a euthyroid state
Study Notes
Thyroid Gland
- Synthesizes 90% T4 and 9% T3
- 0.04% free T4 and 0.4% free T3
- Not active, with long t1/2
Boundaries
- Anteriorly: Carotid sheath
- Laterally: SCM and Carotid sheath
Laryngeal Nerve
- Superior laryngeal nerve
- External laryngeal nerve
- Internal laryngeal nerve
- Recurrent laryngeal nerve
Venous Drainage
- Superior Thyroid Vein from external jugular vein
- Middle thyroid vein from external jugular vein
- Inferior thyroid vein from brachiocephalic and subclavian vein
Organification
- Tyrosine residues of thyroglobulin are iodinated
- Inhibited by large intake of iodine (>150mcg/day) and thioamides (hyperthyroidism therapy)
- Produce monoiodotyrosine (MIT) and diiodotyrosine (DIT) residues
Coupling
- DIT + MIT = T3
- DIT + DIT = T4
Storage
- Along with thyroglobulin
Exocytosis and Proteolysis
- Release of T4 and T3
Conversion of T4 to T3 in Peripheral Tissue
- T4 vs. T3
Thyroid Function Test
- Thyroid hormone (TH) production and secretion regulated by pituitary's TSH
- Two thyroid hormones: T4 and T3
- As TH decreases, TSH increases to stimulate thyroid gland
- Increased TSH levels stimulate gland size and vascularity
Laboratory
- Most sensitive test for hypothyroidism is elevated serum TSH
- In subclinical hypothyroidism, TSH is elevated, whereas free T4 remains normal
Thyrotoxicosis
- Clinicopathologic and biochemical syndrome resulting from excessive thyroid hormone concentrations
- Can be overt or subclinical
- Overt thyrotoxicosis: high serum T4 and T3 concentrations and low serum TSH concentrations
- Subclinical thyrotoxicosis: normal serum T4 and T3 concentrations and low serum TSH concentrations
Pathophysiology
- Unregulated release of T4 and T3 from thyroid gland or ingestion of excessive amounts of T4 or T3
- Can be due to increased T4 and T3 synthesis and release because of intrinsic thyroid disease, excessive TSH, or other thyroid-stimulating hormones
When to Consult an Endocrinologist?
- In children and infants
- In patients in whom it is difficult to render and maintain a euthyroid state
- During pregnancy
- In women planning to conceive
- In cardiac disease
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