ENDOC- THYROID intro Flashcards
The thyroid (Greek thyreos , shield, plus eidos , form) consists of two lobes connected by an_______________
The thyroid gland is located where?
It is located anterior to the trachea between the cricoid cartilage and the suprasternal notch.
What is the weight of thyroid?
The normal thyroid is 12–20 g in size, highly vascular, and soft in consistency.
What is located posterior to the thyroid gland?
Four parathyroid glands, which produce parathyroid hormone (Chap. 353) , are located posterior to each pole of the thyroid.
The _______________ traverse the lateral borders of the thyroid gland and must be identified during thyroid surgery to avoid injury and vocal cord paralysis.
recurrent laryngeal nerves
The thyroid gland produces two related hormones, _____________
thyroxine (T 4 ) and triiodothyronine (T 3 )
T3 and T4 acting through thyroid hormone receptors α and β, these hormones play a critical role in ____________________________
- cell differentiation during development
- and help maintain thermogenic and metabolic homeostasis in the adult.
Thyroid hormones are derived from____________ a large iodinated glycoprotein
Tg,
What happens after secretion into the thyroid follicle, _______________
Tg is iodinated on tyrosine residues that are subsequently coupled via an ether linkage.
________________ cell allows proteolysis and the release of newly synthesized T 4 and T 3 .
Reuptake of Tg into the thyroid follicular
____________ is a critical first step in thyroid hormone synthesis
Iodide uptake
. Ingested iodine is bound to serum proteins, particularly _________. Unbound iodine is excreted in the urine.
albumin
The thyroid gland extracts iodine from the circulation in a highly efficient manner. For example, 10–25% of radioactive tracer (e.g., 123 I) is taken up by the normal thyroid gland over 24 hours; this value can rise to **70–90% in Graves’ disease. **
Iodide uptake is mediated by _________________-, which is expressed at the basolateral membrane of thyroid follicular cells.
NIS
recommended average daily intake of iodine is _____________for adults
150–250 μg/d
recommended average daily intake of iodine is______________for children,
90–120 μg/d
recommended average daily intake of iodine is______________ ____________- for pregnant and lactating women
250 μg/d
. Urinary iodine is _______________in iodine-sufficient populations.
>10 μg/dL
After iodide enters the thyroid, it is trapped and transported to the____________of thyroid follicular cells, where it is oxidized in an organification reaction that involves TPO and hydrogen peroxide.
The reactive iodine atom is added to selected tyrosyl residues within Tg, a large (660 kDa) dimeric protein that consists of 2769 amino acids.
**apical membrane **
The iodotyrosines in Tg are then coupled via an_____________ in a reaction that is also catalyzed by TPO.
Either T 4 or T 3 can be produced by this reaction, depending on the number of iodine atoms present in the iodotyrosines.
ether linkage
After coupling, Tg is taken back into the thyroid cell, where it is processed in_____________to release T 4 and T 3
.
lysosomes
Uncoupled mono- and diiodotyrosines (MIT, DIT) are deiodinated by the enzyme ____________, thereby recycling any iodide that is not converted into thyroid hormones.
dehalogenase
What are the steps in the process of thyroid synthesis?
- Iodide uptake
- •Organification
- •Coupling
- •Storage
- •Release
– most useful physiologic marker of thyroid hormone action
–31kDa
•TSH
–_________ common to other glycoprotein hormones (LH, FSH, hCG)
α subunit:
___________: this subunit is unique to thyroid hormone
–β subunit
what suppress TSH
–Dopamine, glucocorticoids, and somatostatin
____________ stimulates pituitary production of TSH,
Hypothalamic TRH
___________- stimulates thyroid hormone synthesis and secretion.
TSH
Thyroid hormones, acting predominantly through thyroid hormone receptor ___________, feed back to inhibit TRH and TSH production.
β2 (TRβ2)
_____________secreted by the thyrotrope cells of the anterior pituitary, plays a pivotal role in control of the thyroid axis and serves as the **most useful physiologic marker of thyroid hormone action. **
It is a 31-kDa hormone composed of α and β subunits;
the α subunit is common to the other glycoprotein hormones [luteinizing hormone, follicle-stimulating hormone, human chorionic gonadotropin (hCG)], whereas the TSH β subunit is unique to TSH.
TSH,
________________-suppress TSH but are not of major physiologic importance except when these agents are administered in pharmacologic doses.
Dopamine, glucocorticoids, and somatostatin
TSH regulates thyroid gland function through the ______________, a seventransmembrane G protein–coupled receptor (GPCR). Most of these activating mutations occur in the transmembrane domain of the receptor. They mimic the conformational changes induced by TSH binding or the interactions of thyroid-stimulating immunoglobulins (TSI) in Graves’ disease.
TSH-R
Thyroid follicles are formed by thyroid epithelial cells surrounding proteinaceous colloid, which contains_______________.
thyroglobulin
______________- which are polarized, synthesize thyroglobulin and carry out thyroid hormone biosynthesis (see text for details).
Follicular cells,
T4 is secreted from the thyroid gland in about___________excess over T3 (Table 341-2)
.
** twentyfold **
Thyroid hormones bind to what proteins?
- thyroxine-binding globulin (TBG),
- transthyretin (TTR, formerly known as thyroxine-binding prealbumin, or TBPA), and
- albumin.
What is the purpose of the plasma-binding proteins_________________
- increase the pool of circulating hormone,
- delay hormone clearance,
- and may modulate hormone delivery to selected tissue sites.
When the effects of the various binding proteins are combined, approximately 99.98% of T 4 and 99.7% of T 3 are protein-bound.’
________________ may be thought of as a precursor for the more potent T 3.
T 4
T4 is converted to T 3 by the_____________ (see slide 3)
deiodinase enzymes.
T 3 is bound with 10–15 times greater affinity than T4 , which explains its____________-. Though T 4 is produced in excess of T 3 , receptors are occupied mainly by T 3 , reflecting T 4 → T 3 conversion by peripheral tissues, greater T 3 bioavailability in the plasma, and receptors’ greater affinity for T 3 .
increased hormonal potency
Plasma proteins:
_____________________ - carries about 80% of the bound hormones
Thyroxine-binding globulin
• - __________________-carries about 10% of T 4 but little T3
Transthyretin
______________ - binds up to 10% of T 4 and 30% of T3
•Albumin
The deiodinases
- Type I
- Type II
- Type III
Type I deiodinase, which is located primarily in___________, ___________ and ___________ has a relatively low affinity for T 4.
- ** thyroid,**
- liver,
- and kidneys,
Type II deiodinase has a higher affinity for T 4 and is found primarily in the __________, ____________-, ____________ and _________
- pituitary gland,
- brain,
- brown fat,
- and thyroid gland.
Expression of type II deiodinase allows it to________________-, a property that may be important in the context of levothyroxine (T 4 ) replacement.
Type II deiodinase is also regulated by thyroid hormone; hypothyroidism induces the enzyme, resulting in enhanced T 4 → T 3 conversion in tissues such as brain and pituitary
regulate T 3 concentrations locally,
. T 4 → T 3 conversion is impaired by________________
- fasting,
- systemic illness
- or acute trauma,
- oral contrast agents,
- and a variety of medications (e.g., propylthiouracil, propranolol, amiodarone, glucocorticoids).
__________________- inactivates T 4 and T 3 and is the most important source of reverse T 3 (rT 3 ).
Type III deiodinase
_____________- that express type III deiodinase are a rare cause of hypothyroidism in infants.
Massive hemangiomas
Measurement of thyroid hormones
- TSH levels change dynamically in response to alterations of T4 and T3
- finding of an abnormal TSH level must be followed by **measurements of circulating thyroid hormone levels **
Because TSH levels change dynamically in response to alterations of T 4 and T 3 , a logical approach to thyroid testing is to first determine whether TSH is ____________, _________ and _______
suppressed, normal, or elevated.
The finding of an abnormal TSH level must be followed by _____________to confirm the diagnosis of hyperthyroidism (suppressed TSH) or hypothyroidism (elevated TSH).
measurements of circulating thyroid hormone levels
