The thyroid (t.) Is an endocrine gland (that is, a gland with internal secretion) located in the subioid region of the neck, at the front of the trachea (ie under the "Adam's apple"), and is made up of 2 lateral lobes joined by a isthmus. The gland has lateral ratios with the sternocleidomastoid muscle and with the carotid arteries, posteriorly with the laryngeal-recurrent nerve, with the trachea and with the esophagus. Its relationship with the nerve is important for any compression actions that can be exercised (dysphonia).
The t. is vascularized by 2 upper
thyroid arteries, branches of the internal carotid. From an
anatomical-microscopic point of view the fundamental unit is the "follicle",
about 300 microns in diameter, of cubic shape that delimit the follicular cavity,
filled with colloid substance, consisting of thyroglobulin, which includes
iodinated tyrosine residues and represents the form of deposition of thyroid
hormones.
They consist of thyroxine, T4 and triodothyronine, T3. For 65% they are iodine. The intake of daily iodine is therefore essential for the formation of hormones and ranges from 20 to 600-1000 micrograms / day; in areas where iodine is injected, we will therefore have pathologies that go under the name of endemic goiter due to iodine hypertrophy due to iodine deficiency and by TSH stimulation (see below). The thyroid is, in fact, greedy for iodine and capta all iodine available in circulation (iodide trap), by virtue of an energy-dependent active transport pump mechanism. Iodine uptake depends on a hypothalamus hormone, TSH, or thyroid stimulation hormone. The I. captate is, in turn, incorporated into the thyrosine radicals of thyroglobulin, through an oxidation process, catalyzed by a peroxidase system.
This metabolic stage also appears to be modulated by physiological agents and is under the stimulus of TSH. The Iodium will be thus deposited as thyroxine or T4, with 4 molecules of I, or triiodothyronine or T3, within the thyroglobulin molecule. The release of the thyroid hormones, however, will occur through the proteolysis of thyroglobulin by proteases and peptidases, with free T3 and T4. But the true active hormone is T3. Two serum proteins, the TbG or "thyroid-hormone binding globulin" and the other the TbPA, or "thiroid binding prealbumin" are deputy to the vehiculation. The main metabolic transformation of thyroid hormones is achieved through consecutive removal of individual atoms (monodeiodinations) which ultimately lead to total loss of iodic content and biological activity of the molecule.
The regulation of the activity of t. is aimed at maintaining adequate
circulating levels of T3 and T4 and is entrusted to 3 control systems: the first
is constituted by the pituitary release of TSH, "thyroid stimulating hormone",
(in turn controlled by the hypothalamus, with a feed mechanism -back, from the
TRH, "thyrotropin releasing hormone"), the second intratiroideo consists in the
possibility of self-regulation of the release of T3 and T4 as a function of the
levels of intracellular organic iodine; the third, perferric, is represented by
the activity of the microsomal monodeiodinases and by the consequent
transformation of the T4 into T3, biologically more active.
Thyroid US examination: Benign Thyroid Nodule
with Degenerative Changes.
The increase in oxygen consumption and heat production has been related to
thyroid hormones.
These effects depend on the fact that T3 and T4 activate cellular respiration
and metabolism. At the metabolic level, thyroid hormones stimulate
glycogenolysis, neoglucogenesis and have hyperglycemic action; on the metabolism
of lipids have a lipolytic action, through the activity of catecholamines.
Finally in low doses they have a protoanabolic action. On the heart T4 and T3
determine tachycardial action and pump increase; on the digestive tract, an
increase in motility but a reduction in absorption. On the skeletal system, the
action will be the activation of osteoclasts and, therefore, of bone resorption.
Direct actions on mitochondria (cell power plants) lead to an increase in ATP
production and oxygen consumption. It would seem, therefore, that thyroid
hormones rather than a single site of action have multiple and coordinated
docking sites. Once the patient has been examined, as we know, the patient may
present a goiter (see how to visit a diarrhoid patient), ie there is a swelling
in the anterior region of the neck. The endocrinologist, in order to have a
picture of the hormonal structure and the situation of the gland, generally
prescribes the blood and instrumental tests as better specified below:
FT3
FT4
TSH
autoantibodies antimicrosomes, anti-tireoglobulin etc.
ultrasound of the thyroid
thyroid scintigraphy
CT scan of the anterior region of the enlarged neck as appropriate (see eg
thyroid cancer)
thyroid scintigraphy, hypocaptic cold
areas and hypercaptant hot areas
Calculation of basal metabolic rate based on the response of the fasting
patient's oxygen consumption, expressed as a percentage change compared to
normal. The speed of contraction and release of the muscle was apparently used
in the diagnosis of hyper and hypothyroid syndromes.
The finding of low cholesterol values and the increase in calcemia in
hyperthyroidism, and, conversely, the decrease in cholesterol values in
hypothyroidism.
Direct investigations. The measurement of thyroid uptake of radioactive iodine
is one of the most often used techniques. It consists in the determination of
the incorporation of a radiocompound at the thyroid level, at a distance of
various time, usually at the 6th and 24th hour, and is between 5-25% of the
administered dose. If the thyroid gland has reduced activity, the uptake will be
decreased, while in the hyperactivity, it will be increased. Thyroid
scintigraphy, on the other hand, is a survey conducted to evaluate the
distribution of a radioactive iodine dye (radioiodium) in the thyroid parenchyma,
so that hypercaptative areas, through a computer, take conventional coloration
in yellow and red (bright colors) and areas hypocaptants in blue and green; in
this way it will be possible to distinguish the cold nodules, hypocaptants, of
suspicious dysplastic or neoplastic nature (in fact the tumor tissue loses its
function of captation) and hot nodules, or hypercaptant ares (thyroid adenoma,
plummer m., toxic adenoma). Finally, the thyroid hormones will be dosed, the
majority of which, however, is linked to TbG and TbPA. The direct dosage is
possible, however, with radioimmunoassay methods and allows to have the values
of the hormone free or "free", which is the most active from the metabolic
point of view, and therefore fT3 and fT4. The radioimmunoassay of TSH also takes
on significant significance because it allows us to assess whether the gland is
stimulated or not; the values are between 0.5 and 5.0 microUnits / ml; for
example. in the forms with hypothyroidism, due to primary glandular
insufficiency, the levels of TSH are constantly high, while in the forms with
primary hyperthyroidism, for ex. in Plummer Toxic Adenoma, the TSH level will be
null (thyrotoxicosis). The TRH test, instead, will evaluate the hypophyseal
reserve of thyrotropic hormone, which will be reduced both during hypothyroidism
and hypothalamic-hypophyseal insufficiency. The test uses the administration of
TRH which increases the values of TSH between 5 and 20 microUnits / ml in the
euthyroid. The answer is enhanced in the hypothyroid subject. Other important
investigations are the research of anti-thyroglobulin antibodies and
anti-thyroid microsomes, in case of Hashimotoed thyroiditis in a considerable
number of subjects with Basedow's disease and hypothyroidism and of
thyrostimulant immunoglobulins (TSI), typical of Basedow. Compare now the
specific part on thyroid diseases: