Thyroid function Flashcards
1
Q
What is the thyroid gland and what is it responsible for?
A
- Butterfly shape organ located in neck – near the Larynx.
- Responsible for the secretion of thyroid hormones ( T3 and T4 )
2
Q
What is the parathyroid gland and what is it responsible for?
A
– 4 yellowish organ located w/in the thyroid gland – primary responsibility in maintaining calcium levels
3
Q
What is thyroglobulin?
A
main storage site of thyroid hormones. Is the protein precursor to thyroid hormones
4
Q
What is thyrotropin (TSH)
A
regulated thyroid hormones
5
Q
What is T3 and what does it do?
A
- triiodothyronine - Regulates metabolism, growth and development
- 20% of T3 derived from the thyroid gland
- Most come from de-iodinzation of T4 from liver, kidney and muscle
6
Q
What is T4 and what does it do?
A
- thyroxine- major hormone secreted from the thyroid gland
- Most is bound by TBG - FT4 - Only the unbound fraction is biologically active
- Other biologically inactive forms – rT3, MIT, DIT
- MIT and DIT are precursors to T3 and T4
- rT3 comes from peripheral de-iodination of T4
7
Q
What is calcitonin and what does it do?
A
– Not much known about physiologic role in humans
- Release is stimulated by increasing circulating calcium levels in blood
- Serves as a antagonist to PTH – inhibits osteoclastic bone activity ( decrease calcium )
8
Q
How is thyroid hormone synthesized in the thyroid gland?
A
- Iodine is ingested in food and water, concentrated in thyroid gland and” trapped”
2.Incorporated into the amino acid tyrosine – ( Thyroglobulin )
3.Concentrated iodine is oxidized and bound to tyrosyl residues on thyroglobulin – catalyzed by thyroid peroxidase ( TPO )
4.MIT ( monoiodothyronine ) and DIT ( Diiodothyronine ) are formed as a result
TPO acts to couple the MIT and DIT to form the T3 and T4 ( active forms of thyroid hormone )
5.TSH stimulates lysosomes to cleave T3 and T4 and release into blood stream
Up to 80% of T4 is further Deiodinated to T3 and rT3 in the liver
T3 is metabolically active, rT3 is not ( when T3 is up, rT3 is down )
9
Q
What carrier proteins is thyroid hormone bound to?
A
- Thyroid binding protein binds as much as 80% of thyroid hormones
- Thyroid binding prealbumin
- Albumin
- Transthyretin
- The remaining amount ( <1/2 % ) of T3 and T4 are free form and metabolically active – free to travel across cell membranes
- Amount of circulating total T3 and Total T4 are affected by circulating binding
10
Q
How does the Hypothalamus-Pituitary-Thyroid Axis – control mechanism for thyroid function?
A
- Operates on neg feedback
1. Hypothalamus – releases TRH ( thyroid stimulating hormone ) – in repsonse to circulating throid hormone
2. Pituitary – releases TSH in response to TRH secretion
3. Thyroid hormone – Release T3 and T4 in response to TSH secretion
4. Circulating T3 and T4 controls release of TSH ( short loop ) and TRH ( long loop )
5. T3 and T4 perform many function – act on liver, heart, bone and CNS
11
Q
How is TSH released?
A
released in pulsating nature and is diurnal
12
Q
When is TSH highest?
A
highest at night
13
Q
How long is the half life is TSH?
A
- half-life is relatively long (50 min 0 compared to other hormones
- Single measurement is adequate for assessing circulating levels
- This is why it is a good biomarker for hypo and hyperthyroidism
14
Q
What are the three generations of TSH testing?
A
1st generation – RIA
2nd generation – Immunoradiometric
3rd generation – Chemilluminescence
15
Q
In what conditions is TSH commonly increased or decreased in?
A
Best initial test for use in ambulatory, non-hospitalized patients
Increased in hypothyroidism
Decreased in hyperthyroidism
16
Q
What is FT4 used for?
A
used in conjunction with TSH ( normal – 0.7-1.8 mg/dL )
17
Q
When would you see an decrease in FT4?
A
FT4 - decreased in hypothyroidism
18
Q
when would you see an increase in FT4?
A
FT4 – increased in hyperthyroidism
19
Q
What is primary thyroid dysfunction?
A
Thyroid is site of defect
20
Q
What is secondary thyroid dysfunction?
A
pituitary is the site of the defect
21
Q
what is tertiary thyroid dysfunction?
A
Hypothalamus is site of defect
22
Q
What are the causes of hyperthyroidism?
A
- Excess thyroid hormone ingestion
- Leakage of stored thyroid hormone
- Excess gland production of hormone
23
Q
what are some causes of hypothyroidism?
A
- Autoimmune
- Iodine deficiency
- Radioactive iodine treatment
24
Q
What is a physical defect due to hypo/hyper thyroidism?
A
Goiter may present in both cases
25
What is the treatment for hypothyroidism?
levothyroxine
26
What is Hashimoto's Thyroiditis?
- Autoimmune condition commonly associated with permanent hypothyroidism
- Is a primary hypothyroidism resulting in insufficient T4 available to tissues
- Cell and antibody mediated destruction of thyroid tissue – goiter may result
- Exact cause is unclear
- 90% of patients will have antibodies to thyroglobulin or thyroid peroxidase
27
What is Graves' Disease?
- Autoimmune disorder characterized by diffuse , toxic hyperplasia
- Common in North America – affect 0.4% of population
- Caused by IgG aby vs Thyroid TSH receptor = overproduction of thyroid hormones
28
What is Euthyroid Sick Syndrome and what are the signs and symptoms?
- Changes in Thyroid tests in presence of Acute and Chronic illness
- Decrease in T3 and FT3, increase in rT3 and normal T4 and TSH
- rT3 elevated due to impaired conversion of T4 to T3 via peripheral deiodination
29
What are the most important glands? for calcium regulation?
Parathyroid and Thyroid glands instrumental in calcium regulation
30
What are the two most important hormones for calcium regulation?
PTH and Vit D 25 (OH ) are 2 most important hormones in Calcium regulation
31
What is the role of PTH on calcium?
synthesized and secreted by parathyroid glands – acts directly on bone and kidney to increase Ca
32
What is vitamin D's role on calcium?
– Endogenous exposure to sign and via foods
- VitD aids in reabsorption of Ca from GI tract
- Regulates calcium and and phosphate metabolism
33
What is calcitonin's role on the parathyroid gland?
- has pharmacological effects but physiological role has not been established in humans
- Released from Thyroid Gland – increases seen in Thyroid cancer
34
What is the bodies response to low calcium?
- Parathyroid will increase secretion of PTH
- Increase reabsorption of Calcium, Decrease PO4 reabsorption
- Mobilize calcium from bone, decrease excretion in urine
- Returns calcium to normal by increasing calcium
35
What is the bodies response to high calcium?
- Secretion of PTH is suppressed, blood Calcium is reduced.
- Decrease reabsorption by kidney and
- decrease mobilization from bone
36
What are the signs of primary hyperparathyroidism?
- Over production of PTH – Usually an adenoma of the parathyroid gland
- Increased PTH, Increased serum Ca, decreased phosphorus, Increased urine calcium, Increased Vit D 1,25 dihydoxy
- most common cause of hypercalcemia
37
What does renal failure cause permanent abnormalities in?
Permanent abnormalities in Ca, PO4, PTH, VitD, bone mineralization and vascular and soft tissue calcification
38
What are the 3 forms of total calcium?
- Calcium bound to Albumin and other proteins
- Calcium
- Calcium unbound or “ free “
39
What is free calcium controlled by?
- Free Calcium is biologically active and tight controlled by PTH and Vit D 1,25 dihydroxy
- Considered best indicator of Calcium status
40
What is the only acceptable anticoagulant for calcium?
Heparin is only acceptable anticoagulant – Citrate, Oxylate and EDTA bind Calcium and will decrease free calcium
41
What are considerations that should be made for calcium samples?
- Handle anaerobically – minimize pH alteration due to metabolism of cells and loss of CO2 ( will increase pH )
- Seal tubes to minimize CO2 loss – St. Mike’s – we wrapped tube with tape
- Prevent lactic acid formation – collect, transport and maintain on ice to prevent this
- NOTE – collecting and storing on ice will increase K – collect separate sample for this
42
What forms of heparin should be avoided for calcium samples?
- Liquid heparin should be avoided – dilutional effect will decrease free calcium
- High concentrated heparin will bind calcium and decrease free calcium
43
What are common preanalytical errors for calcium?
- Prolonged tourniquet use will increase total Calcium, but not free calcium
- Put on just prior to draw, release after 1 min
- Avoid fist pumping – will generate lactic acid and decrease pH
- Others including posture changes have minimal effects
44
How will a decrease in pH affect calcium?
- binding of calcium to albumin and proteins is affected by pH – this alters free calcium
- Decrease in pH will DECREASE ionization and neg charge on albumin and other proteins
- This will cause an DECREASE in calcium binding to proteins and will INCREASE free calcium in sample
45
How will an increase in pH affect calcium samples?
- Increase in pH will INCREASE ionization and neg charge on albumin and other proteins
- This will cause an INCREASE in calcium binding to proteins and will DECREASE free calcium in sample
46
How much does pH affect calcium?
Calcium will change 5% for every .1 change in pH
