Describe the anatomy of the thyroid gland.
- thyroid gland consists of two bulky lateral lobes connected by a relatively thin isthmus,
Describe the anatomical location of Thyroid.
- usually located below and anterior to the larynx.
Describe the histological appearance of the Thyroid.
- The thyroid is divided by thin fibrous septae into lobules composed of about 20 to 40 evenly dispersed follicles,
- lined by a cuboidal to low columnar epithelium,
- and filled with PAS-positive thyroglobulin.
Describe the process of release of thyroid hormone.
In response to hypothalamic factors, TSH (thyrotropin) is released by thyrotrophs in the anterior pituitary into the circulation.
The binding of TSH to its receptor on the thyroid follicular epithelium results in conformational change and activation of the receptor, allowing it to associate with a Gs protein ( Fig. 24-8 ).
Activation of the G protein eventually results in an increase in intracellular cAMP levels, which stimulates thyroid growth, and hormone synthesis and release via cAMP-dependent protein kinases.
The dissociation of thyroid hormone synthesis and release from the controlled influence of TSH-signaling pathways results in so-called ________________
thyroid autonomy and hyperfunction
Homeostasis in the hypothalamus-pituitary-thyroid axis and mechanism of action of thyroid hormones.
Secretion of thyroid hormones (T3 and T4) is controlled by
trophic factors secreted by both the hypothalamus and the anterior pituitary.
_______________stimulate the release of thyrotropin-releasing hormone (TRH) from the
hypothalamus and thyroid-stimulating hormone (TSH) from the anterior pituitary, causing T3
and T4 levels to rise.
Decreased levels of T3 and T4
In the periphery, T3 and T4 interact with the thyroid hormone
receptor (TR) to form a hormone-receptor complex that translocates to the nucleus and
binds to so-called _________________ on target genes to initiate transcription.
thyroid response elements (TREs)
___________________ convert thyroglobulin into thyroxine (T4) and lesser amounts of triiodothyronine (T3).
Thyroid follicular epithelial cells
T4 and T3 are released into the systemic circulation, where most of these peptides are reversibly bound to circulating plasma proteins, such as _____________ and ________________, for transport to peripheral tissues.
thyroxine-binding globulin and transthyretin
What is the purpose of the binding protein for T3 and T4?
The binding proteins serve to maintain the serum unbound (“free”) T3 and T4 concentrations within narrow limits yet ensure that the hormones are readily available to the tissues.
In the periphery, the majority of free T4 is_____________; the latter binds to thyroid hormone nuclear receptors in target cells with tenfold greater affinity than does T4 and has proportionately greater activity.
The interaction of thyroid hormone with its nuclear thyroid hormone receptor (TR) results in the formation of a multiprotein hormone-receptor complex that binds to thyroid hormone response elements (TREs) in target genes, regulating their transcription (see Fig. 24-8 )
deiodinated to T3
. Thyroid hormone has diverse cellular effects, including :
- up-regulation of carbohydrate and
- lipid catabolism and
- stimulation of protein synthesis in a wide range of cells.
- The net result of these processes is an increase in the basal metabolic rate
One of the most important functions of thyroid hormone is its critical role in ________________________ (see below).
brain development in the fetus and neonate
The function of the thyroid gland can be inhibited by a variety of chemical agents, collectively
referred to as _________________.
Because they suppress T3 and T4 synthesis, the level of TSH
increases, and subsequent hyperplastic enlargement of the gland (goiter) follows.
What does the
antithyroid agent propylthiouracil do________________
- inhibits the oxidation of iodide
- and thus blocks production of the thyroid hormones;
- parenthetically, propylthiouracil also inhibits the peripheral deiodination of circulating T4 into T3, thus ameliorating symptoms of thyroid hormone excess (see below).
_____________, when given to individuals with thyroid hyperfunction, also blocks the release of thyroid
hormones but through different mechanisms.
*****Iodides in large doses inhibit proteolysis of
Thus, thyroid hormone is synthesized and incorporated within increasing amounts
of colloid, but it is not released into the blood.
The thyroid gland follicles also contain a population of __________________ which
synthesize and secrete the hormone calcitonin.
This hormone promotes the absorption of
calcium by the skeletal system and inhibits the resorption of bone by osteoclasts.
parafollicular cells, or C cells,
Diseases of the thyroid include conditions associated with ________________
We first consider the clinical consequences
of disturbed thyroid function, then focus on the disorders that generate these problems.