Describes in detail thyroid anatomy and physiology. Discusses different causes of hyper- and hypo-thyroidism alongside clinical presentation, investigations and management. Also discusses clinical information about thyroiditis and thyroid cancers.
Thyroid embryology
Thyroid descends into neck from pharynx
It initially in foetus maintains connection to tongue, this is called thyroglossal duct
(should disappear)
Two remnants of duct in child/adult:
1. Foramen cecum in tongue
2. Pyramidal lobe of thyroid (superior to isthmus)
Can present as Ectopic thyroid: Can present as thyroglossal duct cyst:
Functioning thyroid tissue outside of Midline neck mass, usually painless
the gland that classically moves up when
swallowing or tongue protrusion
Commonly found base of the tongue
Usually discovered in childhood
Presents as a mass in tongue
May contain thyroid cells
Common during puberty and
pregnancy
Detected during times of increased
hormone demand
May be the only functioning thyroid
tissue, leads to hypothyroidism. This
can increase TSH production leading
to growth of the ectopic thyroid tissue
Thyroid hormone:
Thyroid 2
, Made by follicular cells, from iodine
and tyrosine
Thyroid hormone contains iodine,
there are two hormones T3 and T4
Thyroid peroxidase (TPO), catalyses
multiple steps in the production of T3 and
T4 from tyrosine and iodine
1. T3 - Triiodothyronine (contains 3
iodines)
2. T4 - Thyroxine (contains 4 iodines)
T4 is the major hormone produced by
thyroid (90%)
Wolff-Chaikoff effect
T3 is more potent hormone (more
Protective mechanism by body to
potent activator of the thyroid
prevent hyperthyroidism due to
hormone receptor)
excess iodine in diet.
T4 is a prohormone for T3
Inhibits organification (adding iodine
Most T4 is converted to T3 in to thyrosine) if there's excess iodine -
peripheral tissue by 5' deiodinase so less thyroid hormone can be
produced
Thyroxine-binding globulin
Amiodarone contains iodine (iod in
Thyroid hormone has poor water name), can cause hypothyroidism
solubility, most is bound to thyroxine- due to Wolff-Chaikoff effect as there's
binding globulin (TBG) too much iodine
Produced by liver Amiodarone also competitive inhibitor
Less TBG —> Less available T3/T4 of 5'deiodinase, so less conversion of
to tissue T4 —>T3. Causes rise in TSH (as
less T3) ALWAYS CHECK TSH
Pregnancy and oral contraceptive pill
LEVEL BEFORE AMIODARONE
raise TBG levels and therefore total T4
THERAPY.
levels
As oestrogen modifies TBG
molecules, slowing its clearance from
Thyroid 3
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