Thyroid Function Test Flashcards
The thyroid gland produces two related hormones
thyroxine (T4) and
triiodothyronine (T3)
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 in Greek
(Greek thyreos, shield, plus eidos, form)
The normal thyroid size
12–20 g in size, highly vascular, and soft in consistency.
Structure traverse the lateral
borders of the thyroid gland and must be identified
during thyroid surgery to avoid injury and vocal cord
paralysis
The recurrent laryngeal nerves
The thyroid gland develops from
the floor of the primitive pharynx
during the third week of gestation. The developing gland migrates
along the thyroglossal duct to reach its final location in the neck.
Thyroid hormone
synthesis normally begins
11 weeks gestation
Neural crest derivatives from the ultimobranchial body give rise to
Thyroid medullary C cells that produce calcitonin
The C cells are interspersed throughout the thyroid gland, although their density is greatest in
the juncture of the upper one-third
and lower two-thirds of the gland.
Plays a minimal role in
calcium homeostasis in humans
Calcitonin
C-cells are important because
of their involvement in
Medullary thyroid cancer
Thyroid gland development is orchestrated by the coordinated expression of several developmental transcription factors.
(TTF)-1, TTF-2, and paired homeobox-8 (PAX-8)
The thyroid gland consists of numerous spherical follicles composed
of
thyroid follicular cells that surround secreted colloid, a proteinaceous fluid containing large amounts of thyroglobulin, the protein
precursor of thyroid hormones
The thyroid follicular
cells are polarized
basolateral surface is apposed to the bloodstream
and an apical surface faces the follicular lumen.
Increased demand for thyroid hormone is regulated by
thyroid-stimulating hormone (TSH), which binds to its receptor on the basolateral surface of the follicular cells
TSH secreted by
thyrotrope cells of the anterior pituitary
Deiodination leads to production
potent hormone triiodothyronine
(T3) or the inactive hormone reverse T3
TSH is hormone composed of _subunits;
subunit is common to the other glycoprotein hormones (luteinizing hormone, folliclestimulating
hormone, human chorionic gonadotropin [hCG]), whereas the TSH β subunit is unique to TSH
The extent and nature of carbohydrate
modification are modulated
by thyrotropin-releasing hormone
(TRH) stimulation and influence the biologic activity of the hormone.
Thyroid hormones act via negative feedback predominantly through
thyroid hormone receptor β2 (TRβ2) to inhibit TRH and TSH production
The “set-point” in thyroid axis is established by
TSH
Peak TSH secretion occurs
15 min after administration of exogenous
TRH
3 substance suppress TSH
Dopamine, glucocorticoids, and somatostatin
Thyroid hormones are derived from
Tg
Is a critical first step
in thyroid hormone synthesis
Iodide uptake
characterized by mental and growth
retardation and 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. These children are
often born to mothers with iodine deficiency, and it is likely that
maternal thyroid hormone deficiency worsens the condition
Cretinism
Concomitant deficiency may also contribute to the neurologic manifestations of cretinism
selenium
Iodine deficiency
remains the most common cause of preventable mental deficiency
The recommended average
daily intake of iodine
150–250 μg/d for adults, 90–120 μg/d for children,
and 250 μg/d for pregnant and lactating women. Urinary iodine is >10 μg/dL in iodine-sufficient populations
TSH regulates thyroid gland function through the
TSHR, a seven-transmembrane G protein–coupled receptor (GPCR).
Serum half-life of Thyroid hormones
T4 7 days
T3 2 days
Fraction directly from the thyroid
T4 100%
T3 20%
% Protein-bound
T4 99.98%
T3 99.7%
Which is located primarily in thyroid, liver, and kidneys,
has a relatively low affinity for T4
Type I deiodinase
has a higher affinity
for T4 and is found primarily in the pituitary gland, brain, brown fat,
and thyroid gland.
Type II deiodinase
inactivates T4 and T3 and is the most important
source of reverse T3 (rT3), including in the sick euthyroid syndrome
Type III deiodinase
In the sick euthyroid syndrome, especially with
hypoperfusion, inactivated in muscle and liver
type III deiodinase
