Ch 2 Thyroid Flashcards
What is the thyroid gland?
An endocrine gland (which releases hormones that go directly into the bloodstream)
What does the thyroid gland do?
-Synthesizes, stores + secretes thyroid hormones
-Functions to control the BMI (basal metabolic rate)
What shape is the thyroid?
Butterfly shaped (has 2 lobes + an isthmus)
Where is the thyroid located?
Lower neck
What is the echo texture of the thyroid?
-Very homogeneous (when normal)
-Medium-high level
-Has a fibrous capsule
What is the average size of the thyroid?
Length: 4-6cm
AP: 1.3-1.8cm
Isthmus: 4-6mm
List the 4 main arteries supplying the thyroid?
-Paired superior thyroid arteries: arise off the ECA
-Paired inferior thyroid arteries: arise from the thyrocervical trunk of the subclavian artery
(sometimes the thyroid ima artery as well - it is an anomaly that only some pt’s have)
How many veins drain the thyroid plexus?
3: the superior, middle + inferior thyroid veins
The superior + middle thyroid vein come off of which vein?
The internal jugular vein (IJV)
The inferior thyroid veins come off of which vein?
The brachiocephalic vein (BCV)
What are the 3 thyroid hormones?
-T3 (triiodothyroine)
-T4 (thyroxine)
-Calcitonin (thyrocalcitonin)
Which thyroid hormones m/c affect the body’s metabolism?
T3 + T4
How is T3 + T4 made?
Follicular cells process iodine to secrete T3 + T4, this increases metabolism
Is there more T3 or T4 in the bloodstream?
-There is more T4
-The body converts T4 into T3
-T3 is more potent + is produced in lesser quantity
Calcitonin is secreted by what type of cells?
Parafollicular cells (c-cells), this lowers blood calcium levels
What are follicles?
-Many small sacs throughout the thyroid
-They are lined with follicular cells and contain colloid in the center
How are thyroid hormones made?
-Iodine binds with thyroglobulin (a protein) within the colloid part of a follicle to form thyroid hormones
-The follicular cells reabsorb thyroglobulin + chemically processes T3 + T4
Do thyroid hormones circulate freely or when bound to protein (TBG)?
Both!
What are thyroid lab tests typically testing for?
-Both free + total T3, T4 + TSH
-Typically testing for conditions such as hyperthyroidism + hypothyroidism
When would calcitonin be tested?
Specifically for cancers
What does euthyroid mean?
Normal lab values
What is thyrotropin-releasing hormone (TRH) released from?
The hypothalamus
What is thyrotropin-stimulating hormone (TSH) released from?
The anterior pituitary gland
What does TSH do?
Arranges thyroid hormone production
TSH is regulated by what?
T3, T4 + TRH
Explain the negative feedback loop of the thyroid in 5 steps?
- Decrease in thyroid hormone causes decreased BMR
- Decreased BMR stimulates release of TRH
- TRH induces the release of TSH
- Thyroid releases T3 + T4
- BMR returns to normal
What is the earliest endocrine gland to form in the human embryo?
The thyroid
When is the thyroid fully developed?
By the end of the 1st trimester
Development of the thyroid begins at what level?
Level of the primitive pharynx
(corresponds to the base of the tongue in adults)
As the thyroid develops, the cells migrate downwards in the neck through a canal called what?
The thyroglossal tract/duct
(this duct m/c disappears once the thyroid reaches its final position in the neck)
What is athyrosis + what is it associated with?
-The most severe abnormality of the thyroid
-Associated with cretinism or congenital hypothyroidism
What is a pyramidal lobe?
-The m/c variation of the thyroid
-Typically extends upward from the isthmus
-Can arise from either lobe
What is the m/c ectopic location for the thyroid?
Lingual (very rare tho)
List 3 other thyroid variations that exclude athyrosis, pyramidal lobe + ectopic locations?
-Absence of isthmus
-Absence of 1 lobe (results in compensatory hypertrophy)
-Continuity of 1 lobe to the other (no distinguishable isthmus)
How common is thyroid pathology?
Very!!!
How does the thyroid look when compared to the surrounding muscles?
Homogeneous + echogenic to muscles
What are the 3 main hormones produced by the thyroid?
T3, T4 + calcitonin
What is the m/c variation of the thyroid?
Pyramidal lobe
