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Endocriknowlogy > Thyroid: Thyroid Gland Physiology and Testing > Flashcards

Thyroid: Thyroid Gland Physiology and Testing Flashcards

1
Q

Normal thyroid size

A

12-20 grams

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2
Q

Location of recurrent laryngeal nerves in relation to the thyroid gland

A

The recurrent laryngeal nerves traverse the lateral borders of the thyroid gland

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3
Q

The thyroid gland develops from the floor of the primitive pharynx during the ___ week of gestation

A

Third week

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4
Q

Thyroid hormone synthesis normally begins at about ___ weeks gestation

A

11 weeks

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5
Q

The density of C cells are greatest in what location (which produce calcitonin)

A

Juncture of the upper 1/3 and lower 2/3 of the thyroid gland

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6
Q

Most useful physiologic marker of thyroid hormone action

A

TSH

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7
Q

Which TSH subunit is common to other glycoprotein hormones (LH, FSH, hCG)?

A

Alpha subunit

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8
Q

Which TSH subunit is unique to TSH?

A

Beta subunit

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9
Q

Predominant thyroid hormone receptor through which thyroid hormones act to induce negative feedback

A

Thyroid hormone receptor B2 (TRB2)

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10
Q

Major positive regulator of TSH synthesis and secretion

A

TRH

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11
Q

Peak TSH secretion occurs __ mins after administration of exogenous TRH

A

~15 minutes

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12
Q

Time at which thyroid hormones are at their highest level

A

Night

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13
Q

Plasma half-life of TSH

A

50 minutes (long)

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14
Q

Substances that can suppress TSH (but not of major physiologic importance)

A

Dopamine
Glucocorticoids
Somatostatin

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15
Q

Critical first step in thyroid hormone synthesis

A

Iodide uptake

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16
Q

Iodide uptake is mediated by which transporter?

A

Sodium-iodide symporter (NIS)

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17
Q

Percentage of radioactive tracer taken up by the normal thyroid gland over 24 hours in an iodine-replete state

A

10-25%

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18
Q

Percentage of radioactive tracer taken up by the thyroid gland in Graves’ disease

A

70-90%

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19
Q

Transporter that mediates iodine efflux into the lumen

A

Pendrin

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20
Q

Mutation of the pendrin gene causes this syndrome which is characterized by defective organification of iodine, goiter, and sensorineural deafness

A

Pendred syndrome

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21
Q

Term used to denote the mental and growth retardation that occurs when children who live in iodine-deficient regions are not treated with iodine or thyroid hormone to restore normal thyroid hormone levels during early life

A

Cretinism

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22
Q

Most common cause of preventable mental deficiency

A

Iodine deficiency

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23
Q

Recommended dietary allowance (RDA) for pregnant and breastfeeding women

A

220 mcg iodine per day for pregnant women
290 mcg iodine per day for breastfeeding women

(WHO recommends daily iodine intake of 250 mcg in pregnancy and lactation)

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24
Q

ATA recommendation for all pregnant and breastfeeding women in the US and Canada regarding iodine content of prenatal multivitamins

A

150 mcg iodine per day

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25
Q

Level of urinary iodine in iodine-sufficient populations

A

> 10 mcg/dL

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26
Q

Subsequent stages in thyroid hormone synthesis after iodide uptake

A

Organification, Coupling, Storage, and Release:

Iodide enters thyroid

Iodide is trapped and transported to the apical membrane of thyroid follicular cells

Organification reaction catalyzed by TPO and hydrogen peroxide (produced by dual oxidase (DUOX) and DUOX maturation factor (DUOXA)) = Iodine

Iodine atom is added to tyrosyl residues within Tg = Iodotyrosines

Iodotyrosines are coupled via an ether linkage, catalyzed by TPO = T4 or T3

Tg is taken back into the thyroid cell and processed in lysosomes to release T4 and T3

Uncoupled mono- and diiodotyrosines (MIT, DIT) are deiodinated, thereby recycling any iodide

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27
Q

Thyroglobulin is a large (660 kDa) dimeric protein that consists of how many amino acids?

A

2769

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28
Q

Enzyme responsible for deiodinating MITs and DITs

A

Dehalogenase

29
Q

TSH-R is what type of receptor?

A

Seven-transmembrane G protein-coupled receptor (GPCR); TSH-R is coupled to the alpha subunit of stimulatory G protein

30
Q

5 factors that alter thyroid function in pregnancy

A

1) Transient increase in hCG during 1st trimester - weakly stimulated the TSH-R - reciprocal fall in TSH that persists into the middle of pregnancy
2) Estrogen-induced rise in TBG during 1st trimester - sustained during pregnancy
3) Alterations in immune system - onset, exacerbation, or amelioration of an underlying autoimmune thyroid disease
4) Increased thyroid hormone metabolism by the placenta
5) Increased urinary iodide excretion - can cause impaired thyroid hormone production in areas of marginal iodine sufficiency

31
Q

Usual level of thyroid hormones during pregnancy

A

Total T4 and T3 levels are about 1.5x higher throughout pregnancy, but the free T4 progressively decreases so that third trimester values in healthy pregnancies are often below the nonpregnant lower reference cutoff

32
Q

Thyroid hormone requirements are increased by up to __% during pregnancy in levothyroxine-treated hypothyroid women.

A

45%

33
Q

> or < for T4 and T3

Total hormone
Fraction of hormone in unbound form
Unbound hormone
Production rate (including peripheral conversion)
Intracellular hormone fraction
Relative metabolic potency
A

Total hormone: T4>T3

Fraction of hormone in unbound form: T4T3

Relative metabolic potency: T4

34
Q

Serum half-life of T4 and T3

A

T4 - 7 days

T3 - 2 days

35
Q

Fraction of T4 and T3 that is directly from the thyroid

A

T4 - 100%

T3 - 20%

36
Q

Plasma protein that carries about 80% of the bound thyroid hormones

A

Thyroxine-binding globulin (high affinity; low concentration)

37
Q

Plasma protein that has low affinity for thyroid hormones but has a high plasma concentration

A

Albumin

38
Q

What are increased during pregnancy and intake of estrogen-containing contraceptives - unbound or total T3 and T4? Why?

A

Total T3 and T4 levels ONLY. Reason: TBG levels are elevated by estrogen, which increases sialylation and delays TBG clearance.

39
Q

Is L-thyroxine requirement increased or decreased in women who are pregnant or on estrogen treatment?

A

Increased (because of increased TBG levels)

40
Q

Expected TFT in euthyroid hyperthyroxinemia / familial dysalbuminemic hyperthyroxinemia (FDH)

A

Increased total T4 and/or T3
Normal FT4 and FT3
Normal TSH

(caused by mutations in TBG, TTR, and albumin)

41
Q

Type I deiodinase

  • affinity for T4?
  • location?
A

Type I deiodinase

  • relative low affinity for T4
  • located primarily in thyroid, liver, kidneys
42
Q

Type II deiodinase

  • affinity for T4?
  • location?
A

Type II deiodinase

  • higher affinity for T4
  • located primarily in pituitary gland, brain, brown fat, thyroid gland
43
Q

Factors that can impair T4 to T3 conversion

A

Fasting, systemic illness or acute trauma, oral contrast agents, medications (PTU, propranolol, amiodarone, glucocorticoids)

44
Q

Type III deiodinase

  • function?
  • location
A

Type III deiodinase

  • Inactivates T4 and T3
  • Most important source of reversed T3
  • Expressed in human placenta (not active in healthy individuals)
  • Activated in muscle and liver in sick euthyroid syndrome
45
Q

Circulating thyroid hormones enter cells by _____ diffusion and act primarily through _____ receptors.

A

Passive diffusion

Nuclear receptors

46
Q

Expression of thyroid hormone receptors

A

TR alpha
- brain, kidneys, gonads, muscle, heart

TR beta
- pituitary, liver

47
Q

Mechanism of thyroid hormone receptor activation

A

1) T4 or T3 enters the nucleus
2) T3 binding dissociates CoR from TR
3) Co-activators are recruited to the T3-bound receptor
4) Gene expression is altered

Thyroid hormone receptor and retinoid X receptor form heterodimers that bind specifically to thyroid hormone response elements in the promoter regions of target genes

In the absence of hormone, TR binds co-repressor proteins that silence gene expression

48
Q

Why is T3 more potent than T4?

A

T3 is bound with 10-15 times greater affinity than T4.

49
Q

Classical forms of thyroid hormone resistance are caused by mutations in the ___ gene.

A

TR beta (autosomal dominant)

50
Q

Expected TFT in classical form of thyroid hormone resistance (TR beta)

A

Increased unbound thyroid hormone levels

Normal TSH

51
Q

Expected TFT in RTH with mutation in TR alpha gene

A

Low or normal T4
Normal or elevated T3
Normal TSH

52
Q

The isthmus is attached to which part of the thyroid lobes

A

Lower one-third

53
Q

Normal thyroid weight

A

12-20 g

54
Q

Diagnostic test that provides the most accurate measurement of thyroid volume and nodularity

A

Ultrasound

55
Q

In which part of the thyroid gland can we appreciate a bruit or thrill?

A

Supero- or inferolaterally (over the insertion of the superior and inferior thyroid arteries)

56
Q

Factors that can decrease TBG binding

A

Androgens, nephrotic syndrome

57
Q

Most common cause of an elevated TSH level

A

Hypothyroidism

58
Q

Other possible causes of subnormal TSH levels aside from thyrotoxicosis

A

First trimester of pregnancy (due to hCG secretion)

After treatment of hyperthryoidism (because TSH can remain suppressed for several months)

Certain medications (high doses of glucocorticoids or dopamine)

Intake of biotin supplements <18 hours prior to blood draw

59
Q

Primary work-up for autoimmmune thyroid disease

A

TPO antibodies (positive in almost all patients with autoimmune hypothyroidism and up to 80% of those with Graves’ disease)

60
Q

Elevated levels of TSH receptor antibody (TRAb) in the setting of clinical hyperthyroidism reflects _____ of TSIs.

Presence/Absence?

A

Presence

61
Q

Serum Tg levels are increased in all types of thyrotoxicosis except __________.

A

Thyrotoxicosis factitia

62
Q

Tg levels are particularly increased in this condition

A

Thyroiditis

63
Q

Main role for Tg measurement

A

Follow-up of thyroid cancer patients

64
Q

Radionuclide uptake is _____ in case of excessive circulating exogenous iodine (e.g., from dietary sources of iodinated contrast dye).

High/low?

A

Low, even in the presence of increased thyroid hormone production

65
Q

Thyroid scintigraphy should be performed if the serum TSH is

a. low
b. normal
c. high

A

a. Low

66
Q

TRUE OR FALSE: Functioning or “hot” nodules are most likely malignant, and FNA biopsy is indicated.

A

FALSE

67
Q

TRUE OR FALSE: The vast majority of thyroid nodules do not produce thyroid hormone (“cold” nodules) and these are more likely to be malignant (~5-10%).

A

TRUE

68
Q

Minimum size of cysts and nodules that can be detected by ultrasound

A

> 3mm

69
Q

The only condition associated with euthyroid hyperthyroxinemia that has X-linked transmission

A

Familial excess of TBG

Endocriknowlogy (9 decks)

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