The Thyroid Flashcards
What are the three main functions of the Thyroid
- Critical for Growth and Development
- Maintains Metabolic Activity and O2 requirements
- Regulates Lipid and Carbohydrate metabolism and thus body weight
Control of Thyroid Hormones is from
Hypothalamus Pituitary Axis
Embryology of the Thyroid
Endodermal origin from pharyngeal gut cia thyroglossal duct- abnormal descent - base of tounge→ mediastinum
- Week 4: appears by foramen caecum
- Week 5: Thyrohypoglossal duct breaks down and the gland descends
- Week 7: Migrates anterior to the Trachea
- Week 10: Thryohypoglossal Duct dissapears
- Week 12-20: Functional Thyroid formed @ 12w but independent by 20-26w!
By 20weeks twi halves have joined by isthmus measuring 4cm long and 2cm wide
What is a common screening test for Thyroid function that is done at birth?
Heel-prick test of Thyroxin hormone (T4)
A T4 test measures the blood level of the hormone T4, also known as thyroxine, which is produced by the thyroid gland and helps control metabolism and growth. The T4 test is performed as part of an evaluation of thyroid function.
Positional anatomy of the Thyroid
Deep to Sternohyoid muscles
Anterior to Recurrent laryngeal nerve and Tracheal rings 2/3
Left in the oesophagus
Supplied by the superior thyroid artery (external carotid) and inferior Thyroid artery (subclavian)
ANS supply and a rich capillary supply
What can you see in a histology slide of the Thyroid?
Colloid: gluey goo that is viscous and clear-coloured, how the produced hormone is stored in lumen (made mostly of Tg)
Thyroglobulin (Tg): protein produced by the follicular cells of the thyroid and used entirely within the thyroid gland.
Thyroglobulin protein accounts for approximately half of the protein content of the thyroid gland.
Follicle is formed by single layer of hormone producing cuboidal epithelium cells
*when TSH is secreted, cells become columnar and Lumen side becomes scalloped due to endocytosis of hormone containing colloid!
Whats the Clinical Relevance of Thyroglobulin
When a patient has Thyroiditis, Tg is released “dumped” into lumen > to the blood stream and can be detected.
This allows us to differentiate from Thyroid cancer, goitre and even factitious thyrotoxicity (patients that are faking it and injesting thyroxine)
What does it mean if you do a thyroidectomy because of Thyroid cancer, but Tg is still being measured.
They could have recurrent cancer
Describe Thyroid Follicular Cells
They are simple cuboidal epithelium and are arranged in spherical follicles surrounding colloid.
They have thyrotropin receptors on their surface, which respond to thyroid-stimulating hormone.
Responsible for the production and secretion of thyroid hormones thyroxine (T4) and triiodothyronine (T3).
Purpose of a Fine Needle Aspiration of thyroid nodules?
procedure used to detect cancer in a thyroid nodule or to treat thyroid cysts.
Parafollicular (c) cells of the thyroid produce….?
Calcitonin.
There’s no calcitonin post-op of a thyroidectomy.
BUT in the aggressive medullary thyroid cancer these cells are mutated and large amounts of calcitonin is secreted into the blood stream.
Dietary Iodine
Iodineis injested and stored/concentration in the thyroid.
Humans need a minimum 150mg iodine/day to make hormones. If the thyroid doesn’t have enough iodine to do its job, feedback systems in the body cause the thyroid to work harder.
It used to be in milk vats to sterilse the produce, but no longer done, and iodine levels are now falling. → GOITRE (compensatory enlargment of the thyroid)
Found in salt, soy products and fish
Iodine gets into the Thyroid via _______
“Trapping”
- Iodine gets trapped by a Sodium-iodide Symporter (NIS) and oxidised to iodide (2 Na+ Actively transported allows entry of 1 iodine molecule)
- Rapidly incorperated into tyrosine molecules in follicular cells then incorperated into Tg
- In the lumen it forms is catatlysed by thyroid peroxidase to mono- and di- iodotyrosine (MIT and DIT) and stored in the lumen as large Tg molecule
- Under the action of TSH the lumen membrane endocytoses Tg and releases T4 and T3 from TG into capillaries. Any unused MIT and DIT are recycled.
See pg 166
Iodine as a Supplement
- Drugs; amioarone (75mg/tablet which is WAY TOO MUCH → iodine induced overactive thyroid which is hard to manage
- Radioactive Iodine: Can also treat Thyroiditis and thyrotoxicsis and cancer but may cause dry eyes/mouth as a side effect.
Describe the Secretion of Thyroid Hormone
90% of the thyroid hormone produced is T4 with the remaining being T3 (and small amounts of reverse rT3)
T4 is converted in tissue (liver + kidney) to T3 which is the active hormone at receptors.
Most is bound to proteins such as thyroid-binding globulin and albumen which acts as a buffer against acute changes in T4 for iodine recycling.
Only 0.3% T3 is free hormone
Which thyroid hormone is produced during illness?
In acute illness the conversion of T4 to T3 is reduced (often in the liver) and leads to a ‘sick eurthyroid state with “low” T3 and “normal” TSH (as T4 is still being produced)
(rT3 is high and produced when sick)
Supply of Thyroid hormones
Large store of thyroid hormone (T4 and T3) is incorperated into Tg (50day supply)
Thyroid hormone release involves endocytosis of TG in the colloi.
T4: main hormone in plasma, and only made from thyroid
T3: Most is in the bloodstream, and onlt 20% is from thyroid
T3/4 bind to tissue receptors which have alpha and beta subunits and these are variably expressed indifferent tissues.
HCG rises in pregnancy, goes to Thyroid and can be accepted at TSH receptor leading to______________
Slightly higher T3 levels during pregnancy
If the T3 and T4 are very high, this means
as you would expect the bodys own thyroid to turn TSH off, then there must be Low TSH , and there’s an issue of an overactive thyroid.
Could be due to thyrotoxicosis
If the T4 is low.
T4 low and TSH high: hypothyroidism
T4 low and TSH normal: then the thyroids been removed or there’s a pituitary tumour
Whats a ‘normal’ level of TSH
People have differing levels that are normal to each individual.
These are controlled via negative feedback.
What do you need to think about clinical post-thyroidectomy>
You will need to take thyroid hormone replacement therapy to manage hypothyroidism after surgery
As there’s NO T3 being produced any more, you need to take an oral dosing of T4 (predisposes T3).
Describe the signs and symptoms of Thyrotoxicosis
Signs and ysmptoms are different in every age group.
There will be major changes in mood: anxiety, depression, sleep disorders or even pyschosis.
Eye signs: lid retraction, periorbital oedema and very red sclera
Causes of Thyrotoxicosis (excess of any thyroid hormone)
- Graves DIsease
- Multinodular Goitre: hypefunctioning regions of the thyroid gland that aren’t suppressed by circulating hormnones
- Thyroiditis
- Drugs: amiodarone, iodine, Thyroxine
- Toxic Nodule
Primary Thyrotoxicosis
Most common is from Graves Disease
**Secondary hyperthyroidism is the term used when the thyroid gland is stimulated by excessive thyroid-stimulating hormone (TSH) in the circulation
On the other hand, what are the symptoms of Hypothyroidism (underactive thyroid)?
- Weight gain
- Cold
- Constipation
- Hair loss and dry skin
- Tired
- Oedema
Low T4 and T3, high TSH
Clinical Points
- Increases in TSH → goitre
- Mutant TSh receptor or constitutively activated TSH receptor leads to to adenoma formation and can cause thyrotoxicosis: a ‘hot nodule’
- Autoantibodies to the TSH receptor causes ongoing stimulation and thyrotoxicosis= graves disease.
- TSH receptor antibody levels can be measure (that drive T4 and T3 production) as well as others (against thyroid peroxidase) to moniter degree of autoimmunity.
- TSH receptor antibody levels can be measure (that drive T4 and T3 production) as well as others (against thyroid peroxidase) to moniter degree of autoimmunity.
Most common cause of hypothyroidism
Hashimoto’s disease (autoimmune disease against the thyroid).
Most comon form of hypothyroidism
High TSH, low T4 and +ve TPO antibodies.
Adult onset slow
Can affect all organ systems
