F1-THYROID Flashcards
(197 cards)
1
Q
What is the typical weight of the thyroid gland?
A
15-25 grams
2
Q
Where is the thyroid gland located?
A
Lower anterior neck
3
Q
What is the shape of the thyroid gland?
A
Butterfly
4
Q
How many lobes make up the thyroid gland?
A
Two lobes
5
Q
What connects the two lobes of the thyroid gland?
A
Isthmus
6
Q
Where are the parathyroid glands located?
A
Posterior to the thyroid
7
Q
What do the parathyroid glands regulate?
A
Serum calcium levels and the recurrent laryngeal nerves
8
Q
What are the main thyroid hormones?
A
Triiodothyronine (T3) + Tetraiodothyronine/Thyroxine (T4)
9
Q
What are the main functions of thyroid hormones?
A
Regulate body metabolism + neurologic development + heat regulation
10
Q
At what gestational week are measurable amounts of thyroid hormone produced?
A
Week 11
11
Q
What essential element is required for thyroid hormone synthesis?
A
Iodine
12
Q
What can iodine deficiency cause?
A
Hypothyroidism
13
Q
What are the consequences of hypothyroidism in newborns?
A
Mental retardation + cretinism + neurological dysfunction + stunted growth + physical deformities
14
Q
What is the recommended daily intake of iodine for adults?
A
150 µg
15
Q
What daily iodine intake can lead to hypothyroidism?
A
Less than 50 µg
16
Q
What is the site of thyroid hormone synthesis?
A
Thyroid follicles
17
Q
What is the main component of colloid in thyroid follicles?
A
Thyroglobulin
18
Q
What is thyroglobulin?
A
Glycoprotein manufactured by thyroid follicular cells rich in tyrosine
19
Q
What is the first step in thyroid hormone biosynthesis?
A
Dietary iodine is converted to iodide and absorbed into the bloodstream
20
Q
What enzyme oxidizes iodide to iodine in the colloid?
A
Thyroid peroxidase (TPO)
21
Q
What is produced when one iodine molecule is added to tyrosine?
A
Monoiodothyronine (MIT)
22
Q
What is produced when two iodine molecules are added to tyrosine?
A
Diiodothyronine (DIT)
23
Q
What does TPO combine to form thyroxine (T4)?
A
Two DIT molecules
24
Q
What does TPO combine to form triiodothyronine (T3)?
A
MIT and DIT
25
How are T3 and T4 released into circulation?
Proteolysis of thyroglobulin in the thyroid follicular cell after endocytosis
26
What percentage of T4 is metabolized into T3?
0.35
27
What percentage of T4 is metabolized into reverse T3 (rT3)?
0.45
28
How much more metabolically active is T3 compared to T4?
3-8 times
29
What enzyme converts T4 into T3 or reverse T3?
Monodeiodinase/Iodothyronine 5’-deiodinase
30
Where is Type 1 monodeiodinase most abundant?
Liver and kidney
31
What drugs slow the activity of Type 1 monodeiodinase?
Propylthiouracil + glucocorticoids + propranolol
32
Where is Type 2 monodeiodinase found?
Brain and pituitary gland
33
Maintains constant triiodothyronine (T3) levels in the CNS
Type 2 monodeiodinase
34
How is Type 2 monodeiodinase activity regulated?
Decreased when thyroxine levels are high + increased when thyroxine levels are low
35
What percentage of circulating T4 is unbound and available for hormonal activity?
0.0004
36
What percentage of circulating T3 is unbound and available for hormonal activity?
0.005
37
What are the three major thyroid hormone binding proteins?
Thyroxine-binding globulin (TBG) + Thyroxine-binding prealbumin (TBPA or Transthyretin) + Albumin
38
Which protein transports the majority of T4 and T3?
Thyroxine-binding globulin (TBG)
39
What percentage of T4 does TBG transport?
Approximately 70-75%
40
What percentage of T4 does Thyroxine-binding prealbumin (TBPA) transport?
Approximately 20-33%
41
What percentage of T4 does Albumin transport?
Approximately 5-10%
42
Which thyroid hormone binding protein has the highest affinity for T4?
Thyroxine-binding globulin (TBG)
43
How does the abundance of Albumin compare to TBG and TBPA in serum?
Albumin is present at much higher molar concentration than TBPA and TBG but has lower affinity
44
What is the function of thyroid hormone binding proteins?
Transport thyroid hormones in blood + create a physiologic buffer to maintain hormone availability + allow uniform tissue distribution
45
Where is TBG produced?
Liver
46
What is the molecular weight of TBG?
Approximately 54 kDa
47
What is the structure of TBG?
Monomeric acidic glycoprotein
48
When was TBG first recognized as the major thyroid hormone transport protein?
1952
49
What effect do mutations or polymorphisms in TBG, TBPA, or Albumin genes have?
Cause inherited abnormalities leading to euthyroid hyper- or hypo-iodothyroninemia
50
Which thyroid hormone binding protein binds about 75% of serum T3?
TBG
51
Which protein binds more T3 than TBPA?
Albumin
52
What is the relative distribution of T4 among TBG, TBPA, and Albumin?
TBG 75% + TBPA 20% + Albumin 5%
53
What is the relative distribution of T3 among TBG, TBPA, and Albumin?
TBG 75% + TBPA <5% + Albumin 20%
54
What is the half-life of TBG?
Approximately 5 days
55
What is the half-life of TBPA?
Approximately 2 days
56
What is the half-life of Albumin?
Approximately 15 days
57
What are the three major proteins that bind thyroid hormones?
Thyroxine-binding globulin (TBG) + Thyroxine-binding prealbumin (Transthyretin) + Albumin
58
What percentage of T4 does Thyroxine-binding globulin (TBG) transport?
0.7
59
What percentage of T4 does Thyroxine-binding prealbumin (Transthyretin) transport?
0.2
60
What percentage of T4 does Albumin transport?
0.1
61
How is thyroid hormone production regulated?
Hypothalamic-Pituitary-Thyroid (HPT) axis
62
What hormone stimulates TSH release from the anterior pituitary?
Thyrotropin-Releasing Hormone (TRH)
63
What does TSH stimulate in the thyroid gland?
Production and release of T3 and T4
64
How do elevated T3/T4 levels affect TRH and TSH secretion?
Inhibit TRH (hypothalamus) and TSH (pituitary) via negative feedback
65
What are the primary effects of thyroid hormones?
Tissue growth + brain maturation + increased heat production + increased oxygen consumption + increased beta-adrenergic receptor expression
66
What hormone acts as a hypocalcemic agent by inhibiting PTH and vitamin D?
Calcitonin
67
Which cells secrete calcitonin?
Parafollicular cells/C cells
68
What is the most useful test for assessing thyroid function?
Thyroid-Stimulating Hormone (TSH) using 3rd generation assay (s-TSH)
69
What TSH assay sensitivity is used for detecting subclinical diseases?
0.01 mU/L (3rd generation)
70
What TSH level indicates subclinical hyperthyroidism?
Low TSH + normal FT3/FT4
71
What TSH level indicates subclinical hypothyroidism?
Slightly high TSH + normal FT3/FT4
72
What lab results indicate primary hypothyroidism?
Low T4/T3 + High TSH
73
What lab results indicate primary hyperthyroidism?
High T3/T4 + Low TSH
74
What characterizes T4 thyrotoxicosis?
High T4 + normal/low T3 + low TSH
75
What characterizes T3 thyrotoxicosis?
Normal/low T4 + High T3 + low TSH
76
What is measured by Free Thyroxine (FT4)?
Biologically active T4 fraction
77
What FT4 level indicates hypothyroidism?
Decreased FT4
78
What FT4 level indicates hyperthyroidism?
Increased FT4
79
What is thyroglobulin used to monitor?
Thyroid cancer recurrence (tumor marker)
80
What percentage of thyroid cancer patients have antithyroglobulin antibodies?
~25%
81
Which antibody is associated with Graves' disease?
TSH receptor antibodies (TRAb/TSHR-Ab)
82
Which antibody is diagnostic for Hashimoto's thyroiditis?
Thyroid peroxidase (TPO) antibodies
83
What does radioactive iodine uptake (RAIU) assess?
Thyroid metabolic activity
84
What does high RAIU indicate?
Hyperthyroidism (metabolically active thyroid)
85
What does low RAIU indicate?
Hypothyroidism/metabolic inactivity
86
What is the primary use of thyroid ultrasound?
Detect nodules + assess thyroid anatomy
87
What procedure is first-line for evaluating thyroid nodules?
Fine-needle aspiration biopsy
88
What is the purpose of fine-needle aspiration biopsy?
Identify malignancies + avoid unnecessary surgery
89
What percentage of women over 65 experience hypothyroidism?
0.2
90
What are common symptoms of hypothyroidism?
Cold intolerance + fatigue + dry skin + constipation + hoarseness + bradycardia + dyspnea on exertion + cognitive dysfunction + hair loss + weight gain
91
What electrolyte imbalance can hypothyroidism cause?
Hyponatremia
92
What causes hyponatremia in hypothyroidism?
Increased urinary sodium excretion + inappropriate ADH release + water retention
93
What type of anemia is associated with hypothyroidism?
Pernicious anemia + anemia from menorrhagia + malabsorption of iron and folic acid
94
What lipid abnormality is common in hypothyroidism?
Hyperlipidemia
95
What causes hyperlipidemia in hypothyroidism?
Decreased LDL receptor activity + decreased lipoprotein lipase activity
96
What is the most common cause of hypothyroidism?
Chronic lymphocytic thyroiditis (Hashimoto's thyroiditis)
97
What antibody is positive in Hashimoto's thyroiditis?
Thyroid peroxidase (TPO) antibody
98
What is a physical finding in Hashimoto's thyroiditis?
Enlarged thyroid gland (goiter)
99
What are other causes of hypothyroidism?
Thyroid surgery + radiation therapy + radioactive iodine treatment
100
What is the treatment of choice for hypothyroidism?
Levothyroxine (T4)
101
phenomena where large iodine intake acutely inhibits thyroid hormone synthesis
Wolff-Chaikoff effect
102
What are the American Thyroid Association screening recommendations for hypothyroidism?
Initial TSH at age 35 + repeat every 5 years + more frequent with risk factors or symptoms
103
What are risk factors for hypothyroidism screening?
Goiter + family history + lithium use + amiodarone use
104
Constellation of findings from excess thyroid hormone in peripheral tissues
thyrotoxicosis
105
What are symptoms of thyrotoxicosis?
Anxiety + emotional lability + weakness + tremor + palpitations + heat intolerance + perspiration + weight loss
106
What are possible causes of thyrotoxicosis?
Excessive thyroid hormone ingestion + leakage from follicles + excessive production
107
What is the most common cause of hyperthyroidism?
Graves' disease
108
What type of disorder is Graves' disease?
Autoimmune
109
What antibodies are produced in Graves' disease?
TSH receptor antibodies (TRAb) + Thyroid-stimulating immunoglobulin (TSI)
110
What are the features of Graves' disease?
Thyrotoxicosis + goiter + ophthalmopathy + dermopathy
111
What is a classic eye finding in Graves' disease?
Exophthalmos (bulging eyes)
112
What is a classic skin finding in Graves' disease?
Pretibial myxedema
113
What is the familial risk for Graves' disease?
15% have close relatives with the condition
114
How much more likely are women to develop Graves' disease than men?
5 times
115
What are typical lab findings in Graves' disease?
High free T4 and/or T3 + low or undetectable TSH + positive TRAb/TSI + high RAIU
116
Why is RAIU high in Graves' disease?
Metabolically active thyroid gland due to antibody stimulation of TSH receptor
117
What are treatments for Graves' disease?
Beta-blockers + thyroid peroxidase inhibitors (propylthiouracil, methimazole) + radioactive iodine + surgery
118
What is the effect of radioactive iodine treatment for Graves' disease?
Destroys thyroid tissue + usually requires lifelong thyroid hormone replacement
119
When is surgery preferred for Graves' disease?
Thyroid cancer or to avoid eye problems with radioactive iodine
120
Single hyperfunctioning thyroid nodule
toxic adenoma
121
Multiple hyperfunctioning thyroid nodules
multinodular goiter
122
What are toxic adenoma and multinodular goiter common causes of?
Hyperthyroidism
123
What is the cause of toxic adenoma and multinodular goiter?
Autonomously functioning thyroid tissue + receptor mutations
124
What are treatments for toxic adenoma and multinodular goiter?
Surgery + radioactive iodine + medication
125
What drug can induce thyroid dysfunction?
Amiodarone
126
What is the iodine content of amiodarone?
37% of molecular weight
127
How does amiodarone affect thyroid function?
Inhibits hormone production (Wolff-Chaikoff effect) + blocks T4 to T3 conversion + can cause hypothyroidism or hyperthyroidism
128
What percentage of patients on amiodarone develop hypothyroidism?
0.08
129
What percentage of patients on amiodarone develop hyperthyroidism?
0.03
130
Transient changes in thyroid hormone levels due to thyroid inflammation
subacute thyroiditis
131
What are the three forms of subacute thyroiditis?
Postpartum + painless + painful (de Quervain's)
132
What is the most common form of subacute thyroiditis?
Postpartum (5-9% of postpartum women)
133
What is postpartum thyroiditis associated with?
TPO antibodies + chronic lymphocytic thyroiditis
134
Subacute lymphocytic thyroiditis not associated with pregnancy
painless subacute thyroiditis
135
What is another name for painful subacute thyroiditis?
Subacute granulomatous thyroiditis or de Quervain's thyroiditis
136
What are symptoms of painful subacute thyroiditis?
Neck pain + low-grade fever + myalgia + tender diffuse goiter + swings in thyroid function test
137
What lab findings are typical in painful subacute thyroiditis?
Elevated ESR + elevated thyroglobulin + usually absent TPO antibodies
138
Abnormal thyroid function tests in critically ill patients without thyroid dysfunction
nonthyroidal illness (euthyroid sick syndrome)
139
What are typical lab findings in nonthyroidal illness?
Normal or low TSH + low T3 + low T4 + high reverse T3
140
What is the prevalence of clinically apparent thyroid nodules in adult women?
0.064
141
What is the prevalence of clinically apparent thyroid nodules in adult men?
0.015
142
What percentage of normal thyroid glands have nodules on autopsy?
0.5
143
What percentage of thyroid nodules are cancerous?
6-9%
144
What is the first-line diagnostic tool for thyroid nodules?
Fine-needle aspiration with cytologic examination
145
What is the interpretation of low TSH and high free T4?
Primary hyperthyroidism
146
What is the interpretation of low TSH and low free T4?
Secondary hypothyroidism or severe nonthyroidal illness
147
What is the interpretation of high TSH and low free T4?
Primary hypothyroidism
148
What is the interpretation of high TSH and high free T4?
Secondary hyperthyroidism or thyroid hormone resistance
149
What is the prevalence of thyroglobulin antibody in the general population?
0.03
150
What is the prevalence of thyroglobulin antibody in Graves' disease?
12-30%
151
What is the prevalence of thyroglobulin antibody in Hashimoto's thyroiditis?
35-60%
152
What is the prevalence of thyroid peroxidase antibody in the general population?
10-15%
153
What is the prevalence of thyroid peroxidase antibody in Graves' disease?
45-80%
154
What is the prevalence of thyroid peroxidase antibody in Hashimoto's thyroiditis?
80-99%
155
What is the prevalence of thyrotropin receptor antibody in the general population?
1-2%
156
What is the prevalence of thyrotropin receptor antibody in Graves' disease?
70-100%
157
What is the prevalence of thyrotropin receptor antibody in Hashimoto's thyroiditis?
6-60%
158
What is the most common cause of primary hypothyroidism?
Hashimoto's thyroiditis (chronic lymphocytic thyroiditis)
159
What antibodies are commonly positive in Hashimoto's thyroiditis?
Anti-TPO (80-99% of cases) + Anti-Tg (35-60% of cases)
160
What are two common iatrogenic causes of primary hypothyroidism?
Treatment for toxic goiter (subtotal thyroidectomy or radioactive iodine)
161
What is a clue in the diagnosis of iatrogenic hypothyroidism?
History and physical exam (neck scar)
162
How can excessive iodine intake cause hypothyroidism?
Inhibits thyroid hormone synthesis (Wolff-Chaikoff effect)
163
What is useful in diagnosing hypothyroidism due to excessive iodine intake?
History and urinary iodine measurement
164
What is the typical course of subacute thyroiditis?
Usually transient
165
What is a common cause of secondary hypothyroidism?
Hypopituitarism (caused by adenoma
166
What is a rare cause of tertiary hypothyroidism?
Hypothalamic dysfunction
167
What is the pathogenic mechanism of Graves' disease?
Anti-TSHR antibody (stimulates TSH receptor)
168
What is the typical TSH level in Graves' disease?
Decreased
169
What is the typical RAIU in Graves' disease?
Increased
170
What other tests are positive in Graves' disease?
Anti-TSHR positive + TSI positive
171
What is the pathogenic mechanism of toxic adenoma?
Benign hyperfunctioning nodule
172
What is the typical TSH level in toxic adenoma?
Decreased
173
What is the typical RAIU in toxic adenoma?
Increased
174
How is toxic adenoma visualized?
Seen on thyroid scan
175
What is the pathogenic mechanism of toxic multinodular goiter?
Foci of functional autonomy
176
What is the typical TSH level in toxic multinodular goiter?
Decreased
177
What is the typical RAIU in toxic multinodular goiter?
Increased
178
How is toxic multinodular goiter visualized?
Seen on thyroid scan
179
What is the pathogenic mechanism of a TSH-secreting tumor?
Benign pituitary tumor
180
What is the typical TSH level in TSH-secreting tumor?
Normal or increased
181
What is the typical RAIU in TSH-secreting tumor?
Increased
182
What test is used for TSH-secreting tumor?
Pituitary MRI
183
What is the pathogenic mechanism of painful thyroiditis?
Leakage of thyroid hormone
184
What is the typical TSH level in painful thyroiditis?
Decreased
185
What is the typical RAIU in painful thyroiditis?
Decreased
186
What test is positive in painful thyroiditis?
Elevated thyroglobulin (Tg)
187
What is the pathogenic mechanism of postpartum thyroiditis?
Leakage of thyroid hormone
188
What is the typical TSH level in postpartum thyroiditis?
Decreased
189
What is the typical RAIU in postpartum thyroiditis?
Decreased
190
What test is positive in postpartum thyroiditis?
TPO antibody
191
What is the pathogenic mechanism of exogenous hormone thyrotoxicosis?
Excess thyroid hormone medication
192
What is the typical TSH level in exogenous hormone thyrotoxicosis?
Decreased
193
What is the typical RAIU in exogenous hormone thyrotoxicosis?
Decreased
194
What is the pathogenic mechanism of ectopic thyroid tissue?
Metastatic thyroid cancer or struma ovarii
195
What is the typical TSH level in ectopic thyroid tissue thyrotoxicosis?
Decreased
196
What is the typical RAIU in ectopic thyroid tissue thyrotoxicosis?
Decreased
197
How is ectopic thyroid tissue visualized?
Distant metastasis seen on radioactive iodine scan
