THYROID DISORDERS (based on T) Flashcards
1
Q
What connects the two lobes of the thyroid gland?
A
The isthmus.
2
Q
What is the typical size and consistency of the thyroid gland?
A
12-20 g, highly vascular, soft consistency.
3
Q
Where is the thyroid gland located?
A
Anterior to the trachea, between the cricoid cartilage and suprasternal notch.
4
Q
How many parathyroid glands are there and what is their function?
A
Four, located posterior to each pole of the thyroid, and they produce PTH for calcium metabolism.
5
Q
Why must the recurrent laryngeal nerves be identified during thyroid surgery?
A
To avoid injury and prevent vocal cord paralysis, which manifests as hoarseness.
6
Q
From where does the thyroid gland originate during development?
A
The floor of the primitive pharynx at the base of the tongue.
7
Q
What is a lingual thyroid?
A
An ectopic location of thyroid tissue at the base of the tongue, sometimes causing hypothyroidism.
8
Q
What is a thyroglossal duct cyst?
A
A remnant of developmental thyroid tissue in the midline of the neck, which does not cause thyroid disorders.
9
Q
When does fetal thyroid hormone synthesis begin?
A
At the 11th week of gestation.
10
Q
Why is maternal thyroid hormone important for the fetus?
A
It provides hormone support before fetal thyroid function begins and is crucial for congenital hypothyroidism cases.
11
Q
What is the structural and functional unit of the thyroid?
A
The thyroid follicle.
12
Q
What are the key elements that function within the thyroid follicle?
A
Thyroglobulin (Tg), tyrosine, iodine, thyroxine (T4), triiodothyronine (T3).
13
Q
What are the primary functions of follicular cells?
A
Synthesizing thyroglobulin, carrying out thyroid hormone biosynthesis, and producing colloid.
14
Q
What hormone do parafollicular (C) cells produce and what is its function?
A
Calcitonin, which plays a minor role in calcium metabolism.
15
Q
What is the most useful physiological marker of thyroid hormone action?
A
TSH (Thyroid-stimulating hormone).
16
Q
How does the hypothalamic-pituitary-thyroid axis regulate thyroid hormone production?
A
TRH from the hypothalamus stimulates TSH release, which stimulates T3/T4 production. High thyroid hormone levels inhibit TRH and TSH secretion.
17
Q
What is the function of TSH receptors on follicular cells?
A
They respond to TSH to stimulate thyroid hormone production.
18
Q
What is the role of the sodium/iodide symporter (NIS)?
A
It facilitates iodide uptake into thyroid follicular cells and is a target for radioactive iodine therapy.
19
Q
What is the Wolff-Chaikoff effect?
A
A transient inhibition of thyroid iodide organification due to excess iodide levels.
20
Q
What are the primary thyroid hormones and which is more potent?
A
T4 (thyroxine) and T3 (triiodothyronine); T3 is more potent.
21
Q
How is T4 converted into active T3?
A
By deiodinase enzymes 1 or 2, which remove the 5’-iodine.
22
Q
What is reverse T3 (rT3)?
A
An inactive form of T3 produced when T4 undergoes 5-deiodination.
23
Q
What is the most common cause of preventable intellectual disability worldwide?
A
Iodine deficiency.
24
Q
What is Pendred syndrome?
A
A disorder caused by a mutation in the pendrin gene, leading to defective iodine organification, goiter, and sensorineural deafness.
25
What are the major serum-binding proteins for thyroid hormones?
Thyroxine-binding globulin (TBG), transthyretin (TTR), and albumin.
26
Why is free T4 or T3 measured in clinical practice?
Because unbound thyroid hormones are biologically active.
27
Why is levothyroxine used for thyroid hormone replacement?
T4 has a longer half-life than T3, making it more stable for treatment.
28
How do circulating thyroid hormones enter cells?
Passive diffusion or via specific transporters such as MCT8, MCT10, and OATP1C1.
29
Where do thyroid hormones primarily act after entering cells?
In the nucleus.
30
What are some nongenomic actions of thyroid hormones?
Stimulating mitochondrial enzymatic responses and acting directly on blood vessels and the heart through integrin receptors.
31
What is the effect of excess T3/T4 on body weight and metabolism?
Weight reduction and increased heat production due to higher metabolism.
32
What symptoms are associated with excess thyroid hormones?
Weight loss, heat intolerance, and increased sweating.
33
What is the effect of thyroid hormones on catecholamine responsiveness?
Increases cell responsiveness to catecholamines (epinephrine, norepinephrine), leading to symptoms like palpitations and tremors.
34
What is congenital hypothyroidism, and how does it present?
A severe form of hypothyroidism that manifests as cretinism, characterized by short stature and mental retardation.
35
How can thyroid disorders affect female reproductive health?
They can cause menstrual irregularities (hemorrhagia, oligomenorrhea, amenorrhea) and loss of libido.
36
What is the first step in laboratory evaluation of thyroid function?
Determine whether TSH is suppressed, normal, or elevated.
37
What is the key laboratory test for diagnosing hypothyroidism and hyperthyroidism?
TSH (Thyroid-Stimulating Hormone) assay.
38
Why is measuring free thyroid hormones (T4, T3) important?
They correspond to the biologically available hormone pool.
39
What are some factors that influence thyroid hormone protein binding?
Illness, medications, and genetic factors.
40
When should TSH not be used as an isolated test for thyroid function?
In suspected or known hypothalamic or pituitary disease.
41
What conditions can lead to falsely elevated TSH levels?
Severe nonthyroidal illness, pituitary/hypothalamic tumors, thyroid hormone resistance, and assay artifact.
42
What conditions can lead to falsely decreased TSH levels?
Pregnancy (first trimester), medications (glucocorticoids, dopamine, biotin supplements).
43
What antibodies are associated with autoimmune thyroid disease?
Anti-thyroglobulin and anti-TPO antibodies.
44
What antibodies are measured in Graves' disease?
TSH receptor antibodies (TRAb), also called thyroid-stimulating immunoglobulins (TSI).
45
What condition shows increased serum thyroglobulin except in thyrotoxicosis factitia?
Thyroiditis (due to thyroid tissue destruction and release of Tg).
46
What is the significance of thyroglobulin levels in thyroid cancer follow-up?
After total thyroidectomy and radioablation, Tg levels should be <0.2 ng/mL; measurable levels indicate incomplete ablation or recurrent cancer.
47
What is the role of radioiodine uptake (RAI) and thyroid scanning?
To assess thyroid function and distinguish between different causes of hyperthyroidism.
48
What does a thyroid scan show in Graves' disease?
Enlarged thyroid gland with increased, homogeneously distributed tracer uptake.
49
What is a ‘hot’ thyroid nodule, and is it malignant?
A nodule with increased function; almost never malignant.
50
What is a ‘cold’ thyroid nodule, and is it malignant?
A nodule that does not produce thyroid hormone; has a higher likelihood of malignancy (5-10%).
51
What is the best imaging tool for evaluating thyroid nodules?
Thyroid ultrasound.
52
What sonographic features suggest malignancy in thyroid nodules?
Hypoechoic solid nodules, irregular borders, infiltrative borders, and microcalcifications.
53
What is TIRADS used for?
It is a validated risk stratification system for thyroid nodules.
54
What is the most common cause of hypothyroidism worldwide?
Iodine deficiency.
55
What are the most common causes of hypothyroidism in iodine-sufficient areas?
Autoimmune thyroiditis (Hashimoto’s) and iatrogenic causes (treatment of hyperthyroidism).
56
What is primary hypothyroidism, and what lab findings does it show?
Destruction of the thyroid gland leading to ↓ T3, T4 and ↑ TSH due to lack of negative feedback.
57
What is the most common cause of primary hypothyroidism?
Autoimmune hypothyroidism (Hashimoto’s thyroiditis).
58
What is the Wolff-Chaikoff effect?
A temporary inhibition of thyroid hormone production due to excessive iodine intake.
59
What is secondary hypothyroidism, and what are its causes?
TSH deficiency due to pituitary tumors, craniopharyngioma, or pituitary surgery/irradiation.
60
What is tertiary hypothyroidism, and what are its causes?
Deficient TSH stimulation due to hypothalamic lesions or pituitary stalk damage.
61
What is transient hypothyroidism?
Temporary thyroid dysfunction due to thyroid inflammation or recovery from hyperthyroidism treatment.
62
What drugs can induce hypothyroidism?
Amiodarone, lithium, antithyroid drugs, tyrosine kinase inhibitors, immune checkpoint inhibitors.
63
What is Sheehan’s syndrome?
Postpartum pituitary infarction due to excessive bleeding, leading to pituitary hormone deficiencies including TSH.
64
What is the most common congenital cause of hypothyroidism?
Thyroid dysgenesis (absent or ectopic thyroid gland).
65
What is the importance of neonatal screening for congenital hypothyroidism?
Early detection allows for T4 treatment, preventing mental retardation and growth impairment.
66
What are the symptoms of hypothyroidism?
Fatigue, dry skin, cold intolerance, weight gain, constipation, hair loss, hoarseness, menorrhagia, and depression.
67
What are the signs of hypothyroidism?
Bradycardia, myxedema, dry skin, puffy face, delayed reflexes, and serous cavity effusions.
68
What is Hashimoto’s thyroiditis?
An autoimmune thyroid disorder with lymphocytic infiltration, follicular atrophy, and fibrosis.
69
What genetic factors are associated with autoimmune hypothyroidism?
HLA-DR3, DR4, DR5, CTLA-4 gene, and chromosome 21 mutation (Down’s syndrome).
70
What is atrophic thyroiditis?
The end stage of Hashimoto’s thyroiditis, characterized by extensive fibrosis and loss of thyroid follicles.
71
What autoimmune diseases are associated with Hashimoto’s thyroiditis?
Vitiligo, pernicious anemia, Addison’s disease, alopecia areata, type 1 diabetes mellitus.
72
What is the key laboratory test to diagnose hypothyroidism?
TSH levels.
73
What additional tests help determine the cause of hypothyroidism?
Free T4, anti-TPO, and anti-thyroglobulin antibodies.
74
What is subclinical hypothyroidism?
Elevated TSH with normal FT4 levels, often detected before clinical symptoms appear.
75
What is sick euthyroid syndrome?
A condition where thyroid hormone levels appear abnormal due to critical illness, but the thyroid gland is functioning normally.
76
What factors influence individualized patient dosing for hypothyroidism treatment?
Age and weight, cardiovascular health, severity and duration of hypothyroidism, concomitant disease states and treatment.
77
What is the treatment of choice for routine management of hypothyroidism?
Levothyroxine sodium (LT4).
78
What is the typical dosage of Levothyroxine (LT4) for hypothyroidism?
1.6 µg/kg body weight (typically 100-150 µg), taken at least 30 minutes before breakfast.
79
Why is Levothyroxine (LT4) widely used?
It is readily available, can be converted to T3 peripherally, and has a long half-life of 7 days.
80
What should a patient do if they miss a dose of Levothyroxine (LT4)?
They can take the skipped dose the following day.
81
What are the characteristics of T3 (Liothyronine) as a thyroid hormone replacement?
Not readily available, short-acting, given several times a day.
82
When should TSH response be evaluated after initiating thyroid hormone treatment?
Start evaluating after 2 months.
83
How often should TSH be monitored when adjusting thyroid hormone therapy?
Every 6 to 8 weeks until TSH is normalized.
84
When do symptoms of hypothyroidism improve after normal TSH levels are restored?
Typically within 3-6 months.
85
How often should TSH levels be checked once full thyroid hormone replacement is achieved?
Annually, or every 6 months.
86
What is subclinical hypothyroidism?
Mild hypothyroidism with slightly elevated TSH but normal FT4 and FT3.
87
What should be done if TSH levels are >10 mIU/L but the patient is asymptomatic?
Repeat testing.
88
What are the TSH level recommendations for pregnant women?
TSH levels should be normal.
89
When is Levothyroxine recommended for subclinical hypothyroidism?
If the patient is a woman who wishes to conceive, is pregnant, or has TSH >10 mIU/L.
90
What is the normal range for TSH levels?
0.4-4 mIU/L or 0.5-5 mIU/L.
91
What is considered a suppressed TSH level?
<0.1 mIU/L.
92
When should a trial of treatment be considered for subclinical hypothyroidism?
For young or middle-aged patients with symptoms or risk of heart disease.
93
Why is subclinical hypothyroidism associated with coronary artery disease?
It increases the risk of coronary artery disease, which can lead to a heart attack.
94
How is subclinical hypothyroidism diagnosed?
Elevated TSH sustained over 3 months.
95
What is the treatment for subclinical hypothyroidism?
Low-dose Levothyroxine (25-50 µg/day) with a goal of normalizing TSH.
96
What is the target TSH level for maternal hypothyroidism treatment?
<2.5 mIU/L prior to conception.
97
When should thyroid function tests be done during pregnancy?
Immediately after pregnancy is confirmed, every 4 weeks in the first half, and every 6-8 weeks after 20 weeks' gestation.
98
What happens to LT4 doses after delivery?
They return to prepregnancy levels.
99
What factors influence thyroid function during pregnancy?
Increased β-hCG, estrogen, thyroxine-binding globulins (TBG), and immune suppression.
100
What should pregnant women be educated about regarding LT4 intake?
Separate ingestion of prenatal vitamins and iron supplements from LT4.
101
What is myxedema coma?
A state of decompensated hypothyroidism (severe hypothyroidism emergency) with a 20-40% mortality rate.
102
Why does thyroid hormone replacement not take immediate effect in myxedema coma?
Severe hypothyroidism impairs metabolism and hormone activation.
103
What are common causes of myxedema coma?
Poor compliance, post-thyroid surgery, or previously undiagnosed hypothyroidism.
104
In what population does myxedema coma almost always occur?
The elderly.
105
What neurological symptoms can occur in myxedema coma?
Changes in sensorium, disorientation, stupor, and coma in severe cases (TSH levels ~100 mIU/L).
106
What factors can precipitate myxedema coma?
Respiratory impairment (hypoventilation → hypoxia & hypercapnia), sedatives, anesthetics, antidepressants, pneumonia, CHF, MI, GI bleeding, CVA.
107
Why is elective surgery often deferred in patients with hypothyroidism?
Surgical sedation can be dangerous due to difficulty waking from anesthesia.
108
How is Levothyroxine (LT4) administered in myxedema coma?
Given via NGT or per rectum (since there is no IV form).
109
What are the hallmark symptoms of myxedema coma?
Hypothyroidism symptoms + reduced consciousness, seizures, and severe hypothermia (as low as 23°C).
110
What is the initial loading dose of Levothyroxine (LT4) in myxedema coma?
Single IV bolus of 200-400 µg, followed by daily oral dose of 1.6 µg/kg/day.
111
Why is Liothyronine (T3) added in myxedema coma treatment?
T4 to T3 conversion is impaired in myxedema coma.
112
What is the Liothyronine (T3) dosing regimen for myxedema coma?
Initial loading dose of 5-20 µg, followed by 2.5-10 µg every 8 hours.
113
What supportive therapy is needed in myxedema coma?
Correction of metabolic disturbances and parenteral hydrocortisone (50 mg every 6 hours).
114
Why is hydrocortisone given in myxedema coma?
To manage impaired adrenal reserve in profound hypothyroidism.
115
What other treatments may be required for myxedema coma?
Broad-spectrum antibiotics for precipitating infections.
