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MS2 Endocrine > 10/7- Intro to Thyroid Disorders > Flashcards

10/7- Intro to Thyroid Disorders Flashcards

1
Q

What is the embryological origin of the medial portion of the thyroid? Lateral?

A

Medial:

  • Foramen cecum at the base of the tongue
  • This migrates caudally until it reaches the adult resting place
  • Thyroglossal duct may persist in adults

Lateral:

  • Ultimobranchial body (part of 5th branchial arch)
  • Maldescent/non-descent may result in persistent thyroglossal duct
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2
Q

Describe the anatomy of the thyroid

  • Lobes
  • Weight
  • Vasculature
  • Microscopy
A
  • 2 lobes and an isthmus
  • Lobes -> lobules -> follicles
  • Normal weight: 15-25g
  • Vasculature supply from inferior and superior thyroid arteries and thyroidal veins
  • Follicle microscopy: epithelial cells surrounding central colloid
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3
Q

What substance is required for thyroid hormone synthesis?

A

Iodine

  • RDA = 150 ug/d
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4
Q

Why is iodine necessary for thyroid hormone synthesis?

  • How does the thyroid gland get enough?
  • Storage?
A
  • Function: iodine catalyzes iodine organification (attachment) to tyrosine residues of thyroglobulin
  • Iodide trapping: thyroid cells actively transport Iodide from plasma into cytoplasm.
  • This entrapment by the Na-I symporter is rate limiting for thyroid hormone synthesis, and is regulated by the TSH
  • Iodide is stored within the thyroid as thyroglobulin
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5
Q

What are the major synthetic steps in thyroid hormone synthesis?

What regulates these steps?

A

Major synthetic steps are regulated by TSH:

  • Iodine transport (Na-I symporter)
  • Organification (attachment) of iodine by TPO: oxidation of iodide before incorporation into tyrosyl residues.
  • Coupling: Iodine coupled to Thyroglobulin: Monoiodotyrosine, Diiodotyrosine, Tri (T3), Tetra (T4) etc.
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6
Q

What does the 4 refer to in T4?

A

Number of attached iodines

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7
Q

What organ/gland makes TSH?

A

Pituitary (anterior)

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8
Q

What are the active forms of thyroid hormone?

A

T3 and T4

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9
Q

What stimulates TSH release?

A

TRH release by the hypothalamus

Overview

  • Hypothalamus (TRH) ->
  • Anterior pituitary (TSH) ->
  • Thyroid (T3, T4) ->
  • Tissue
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10
Q

Describe negative feedback of the thyroid hormone circuit?

A

T3 and T4 feed back to pituitary and hypothalamus to shut down TSH and TRH, respectively

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11
Q

Where is the problem in primary disorders? Secondary?

A
  • Primary- level of thyroid
  • Secondary- level of pituitary
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12
Q

Thyroid function tests analyze what?

A
  • TSH
  • Total T3, T4 (includes what is bound to protein)
  • Free T3, T4
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13
Q

What is TSH?

  • Produced by what gland
  • Stimulated by
  • Inhibited by
  • Promotes what processes
  • Indicator of what
A

TSH/Thyrotropin

  • Produced by anterior pituitary gland
  • Stimulated by: TRH
  • Suppressed by: T4, T3

Stimulates:

  • Iodine uptake
  • Growth of thyroid gland

Reliable indicator of primary hypothyroidism (elevated levels when low T3/T4)

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14
Q

What may cause elevated levels of TSH?

Depressed levels?

A

Elevations:

  • Primary hypothyroidism
  • TSH producing pituitary tumors (rare; 2ndary hypothyroidism)

Depressions:

  • Hyperthyroidism (common)
  • Hypopituitarism (2ndary)
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15
Q

Is the majority of circulating thyroid hormone T3 or T4? Stats?

A

Majority of circulating hormone = T4 (98.5%)

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16
Q

Total hormone load is influenced by what?

A

Serum binding proteins

  • Albumin (15%)
  • Thyroid binding globulin (70%)
  • Transthyretin (10%)
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17
Q

Describe mechanism of Grave’s disease

  • Levels of T3, T4, TSH
A

Thyroidoxitosis

  • Thyroid gland becomes unregulated; makes a whole lot of T3/T4
  • T3/T4 feed back to pituitary to depress TSH levels (undetectable in assay)
18
Q

Regulation of thyroid hormones is based on what?

A

Free component of thyroid hormone

  • FT4 helps in the assessment of the pt’s true metabolic status
19
Q

What causes elevations in FT4? Decreases?

A

Increased levels:

  • Hyperthyroidism
  • Hyperthyroid phase of thyroiditis (early on, damage -> dump hormones?)

Decreased levels:

  • Hypothyroidism
  • Hypothyroid phase of thyroiditis
  • Euthyroid sick syndrome
20
Q

What can cause increased TBG (thyroid binding globulin)? Decreased TBG?

A

Increased TBG:

  • High estrogen states (pregnancy, OCP, HRT)
  • Liver disease (early)

Decreased TBG:

  • Androgens or anabolic steroids
  • Liver disease (late)
21
Q

Describe T3 resin uptake

  • Evaluation of what?
  • Process
  • High/low uptake indicates what
A
  • Used to evaluate binding proteins
  • Patient’s serum + tracer amount of 125I-T3
  • During incubation the binding proteins in the patient’s serum and the resin compete for the tracer until equilibrium is established
  • Resin separated and counted for radioactivity

- High uptake = low binding protein levels

- Low uptake = increased binding protein levels

22
Q

What Abs directed against the thyroid may result in auto-immune thyroid diseases?

A
  • Anti-microsomal Ab
  • Anti-thyroglobulin Ab
  • Anti-TPO (thyroid peroxidase) Ab
23
Q

What do Abs to the thyrotropin receptor cause?

A

Varied effects

  • Some stimulate thyroid gland function (thyroid stimulating immunoglobulins, TSI)
  • Others inhibit TSH binding (thyroid binding inhibitory Ig, TBII)
24
Q

What are possible results of anti-thyroid antibodies?

A

Depends on the type of Ab, possible:

  • Hyperthyroidism
  • Hypothyroidism
  • Goiter
  • Atrophy
25
Q

What is Hashimoto’s disease (what Abs present)?

A
  • Anti-thyroglobulin Ab
  • Anti-thyroid peroxidase Ab
26
Q

What is Graves’ disease (what Abs present)?

A
  • TSI (thyroid stimulating immunoglobulins)
27
Q

What are thyroglobulin (Tg) levels in disease?

A
  • Increased in ALL thyroid disease
  • Useful for thyroid cancer surveillance post surgery and radioiodine ablation
  • NOT useful for initial thyroid cancer diagnosis
28
Q

What is TRH - Produced by what organ/gland

  • Release profile/timing
  • Downregulated by what
  • Stimulates what
A
  • Produce by hypothalamus
  • Release is pulsatile, circadian
  • Travels through portal venous system to adenohypophysis (anterior pituitary)
  • Stimulates TSH formation
  • Downregulated by T3, T4
29
Q

What are diagnostic tests for thyroid function?

A
  • Thyroid uptake scan (RAIU)
  • Ultrasound of the thyroid
  • Fine needle aspiration
30
Q

Describe RAIU and SCAN

  • Process/mechanism
  • Results/indications
  • Uptake varies by what
A
  • Scintillation counter measures radioactivity after I123 administration.
  • This is used to assess the functional status of thyroid tissue.
  • Hyperfunctioning thyroid tissue shows up as a “hot” scan indicating hyperthyroidism
  • Non-functioning thyroid tissues appears as a “cold” defect (suspicious of malignancy).

Uptake varies greatly by iodine status:

  • Diet
  • Amiodarone, Radio-contrast use
31
Q

In what hyperthyroid symptoms do you have high RAIU?

A
  • Graves’ disease
  • Toxic MN goiter
  • Toxic nodule
32
Q

In what hyperthyroid symptoms do you have low RAIU?

A
  • Thyroiditis
  • Subacute
  • Active Hashimoto’s
  • Hormone ingestion
  • Thyrotoxicosis facititia
  • Hamburger thyrotoxicosis
33
Q

What is seen here?

A

RAIUS Left to right:

  • Toxic MNG
  • “Warm” nodule
  • Hot nodule
  • Cold (photopenic nodule)
34
Q

What is seen here?

A

RAIUS Left to right:

  • Diffuse toxic goiter
  • Thyroiditis
35
Q

What is normal 4 hr RAIU?

A

5-20%

36
Q

Describe the 24 hr results of RAIU and thyroid condition

A

> 25%: Hyperthyroid

20-25%: Equivocal (check TSH)

9-20%: Normal

5-9% Equivocal (check TSH)

< 5%: Thyroiditis

37
Q

What may cause variations in RAIU?

A

Dependent on dietary iodine intake!!

38
Q

What are contradindications for RIAUS (radioactive iodine)?

A
  • Pregnancy (test B-hCG)
  • No ATD (anti-thyroid drugs) for 7d
  • No LT4 for 4d
  • No recent use of iodine or radiocontrast (do NOT want to do contrast CT)
39
Q

When would you want a thyroid ultrasound?

A
  • Evaluate thyroid nodularity: single vs. multinodule goiter
  • Assess size and location of nodules
  • Assess solid vs. cystic nodules
  • Monitor progress of thyroid nodule
  • To guide needle biopsy of nodules
  • To monitor local recurrence of thyroid cancer
40
Q

What are indications for fine needle aspiration?

  • Complications?
A

Indications:

  • Evaluation of thyroid nodule/mass for malignancy

Complications:

  • Infection
  • Hemorrhage
  • Damage to the neurovascular bundle

MS2 Endocrine (25 decks)

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