ES Biochemistry 1

Thyroid Hormones: Synthesis, Regulation, and Action

 The thyroid gland is the largest endocrine gland (as it doesn’t have ducts its hormones are released into
bloodstream).
 Its located on the anterior surface of trachea, it’s butterfly like has two lobes that are connected by narrow
isthmus beneath thyroid cartilage and on its posterior surface are 4 parathyroid glands
 It has rich blood supply in order to secret hormones into the blood stream
 Thyroid hormones are T4,T3, and calcitonin.
 Thyroid hormones regulate metabolism and development:
 Oxygen consumption and basal metabolic rate (BMR), as well as lipid, carbohydrate, and protein
metabolism.
 Neurological development of the fetus
 The normal functioning of the thyroid is dependent on adequate and regular dietary intake of
iodine.
 Iodine is an integral part of T3 and T4.
 Sources of iodine include iodized table salt, seafood, seaweed, and vegetables
 The recommended minimum daily intake of iodine is 150 μg to maintain proper amount of thyroid hormone.

Follicles are the functional units of the thyroid gland

 Thyroid gland is encapsulated with fibrous capsule that send septa to divide the gland into
lobules each lobule contain follicles (circular and have lumen lined with cuboidal epithelium lie
on BM)
 The lumen contain colloid → pretentious material these protein secreted from
the follicular cells (thyrocyte)

 The mammalian thyroid gland (these follicular cells ) biosynthesizes, stores, and secretes two
molecular species of thyroid hormone,
1. thyroxine (T4; 3,5,3′,5′-tetraiodothyronine): inactive should be converted to T3 to function
2. triiodothyronine (T3; 3,5,3′-triiodothyronine) active form.

 Thyrocyte basal end is in contact with the capillary bed surrounding the follicle and the apical end is in contact
with the lumen of the follicle (the apical surface of the thyrocyte is arranged as microvilli and some
pseudopods)
 Parafollicular cells (clear cells) secretes calcitonin (for Ca++ regulation)
 Less in number and are imbedded between follicular cells and the basement membrane
 Thyroid hormone are made from tyrosine
 thyroglobulin (TG): protein made by thyrocyte and secreted into the colloid
 It undergoes processing in the colloid by addition of iodine on tyrosine residue
(iodination) to produce T3,T4, monoiodotyrosine(MIT),Diodotyrosin (DIT)
 We call it organification also (addition of I on organic molecule)
 MIT, DIT are precursor for T3 and T4
 Iodination occur on carbon number 3 or and 5 of the phenol ring of tyrosine
 Adding I on carbon 3 → MIT
 Iodination on carbon 3,5 →DIT
 thyronine→ 2 tyrosine attached by the phenol ring by an either bond
 Thyronine has inner and outer ring
 We number the carbon in the inner ring (1,2,3…)
 The outer with prime (1’,2’,3’…)

 Thyroxine(T4) I on carbon 3,5, 3’,5’

 T3 lack I on carbon 5’ (the functional one)
 Revers T3 →lack I on carbon 5 & inactive form
 So, what determine the activity of thyroid H is the location and the number of iodine

Thyrocyte is specialized to carry out all the steps required for the synthesis and secretion of
T4 and T3
 Thyrocytes have two different sides or poles
- the basolateral: capillary side
- the apical side: facing the lumen
1. Active transport of iodide into the follicular cells of
the thyroid gland.
2. Oxidation of iodide→iodine and its incorporation into
tyrosyl residues within the protein thyroglobulin.
3. Coupling of iodotyrosines within thyroglobulin to
form T4 and T3.
 Oxidation and coupling occur in the luminal side
4. Storage of thyroglobulin as colloid in the lumen of the thyroid follicle.
5. Endocytosis(internalization)of Tg within the colloid back into the thyrocyte in a visicle.
6. Proteolysis of thyroglobulin by fusion with lysosome, releasing T4 and T3, along with
free iodotyrosines and iodothyronines.
7. Secretion of T4 and T3 into the bloodstream by MCT8.
 8. Deiodination of iodotyrosines within the thyroid folliclular cells to recycle iodine and
reuse it in synthesis of thyroid hormone

 Enzymes:

 NIS (sodium iodide symporter) →transporter that facilitates the entry/ trapping of iodide from blood into
thyrocyte
 MCT8 (monocarboxylate 8) →transporter that facilitate transport of T3 and T4 from thyrocyte into blood
circulation
 PENDIN on the apical side →facilitate transport of iodide from the lumen into the follicle follicle to lumen
 TPO (thyroid peroxidase) and DUOX (dual oxidase) →on the apical side catalise oxidation of and organification &
coupling on the luminal side
 To produce Tg that has T3, T4, DIT, MIT
 Last step (deiodination ) catalyzed by :
 dehalogenase →remove I from tyrosine
 deiodinase→ remove iodide from MIT, DIT

Thyroglobulin serves as the thyroid hormones precursor

 Tg it’s a protein encoded by gene that will transcribed into mRNA then
translation into protein by ribosome then packaging in vesicles to golgi
apparatus for glycosylation then it will be sent out to the lumen (apical side)
by exocytosis
 Thyroglobulin (Tg) is a dimeric glycoprotein with a molecular mass of
660kDa; the monomer is 330 kDa
 Approximately 10–11% of Tg’s is carbohydrates (which is needed for its
function).
 it has different domains the N-terminal domain, the C-terminal domain, in addition to different domains ( arm,
flap and core)
 It consists of 2748 amino acid residues, of which 134 (4.5%) are tyrosine, the accessible tyrosine (that can be
iodinated to produce iodothyronine then coupling to produce T3 and T4) are actually less than 134.
 About 4 of the accessible tyrosine residues can be used to make T3,T4. The rest of them can
be used as donor to make T3, T4
 In the figure starting from the n-terminal, the blue circle iodination of tyrosine making T4
(there are three)
 We can use one to make T3
 The tyrosine that can be used to make hormone ‘hormonogenic tyrosine’ it make the inner
phenol ring
 The other (the purple) ‘donor’ iodinated tyrosine will form the outer phenol ring
 Synthesis of thyroid hormone requires incorporation of iodine into
tyrosines of thyroglobulin
 in the lumen there are certain action that would assist iodination of tyrosine
residues on Tg

 Oxidation of iodide→the oxidized I will be used for iodination or organification

 If iodination occur on carbon 3 → MIT
 If I is added to 3,5 C →DIT

 Coupling between :
 DIT+DIT → T4
 DIT + MIT →T3

 So we need oxidation, organification and coupling to make T3, T4 on the surface of Tg

 On Tg also we have MIT, DIT, iodotyrosine

The thyroid gland traps iodide from the bloodstream

 Iodide uptake:
 The thyroid gland traps iodide from the bloodstream using a sodium iodide
symporter (NIS) which provide enough I for the synthesis of hormone inside thyrocyte.
 The activity of basolateral Na+-I- symporters, driven by Na+-K+- ATPase,
brings iodide from circulation into the thyrocytes.
 Iodide transport with 2 Na+ to the thyrocyte
 The driving force is an electrochemical sodium gradient generated by Na+/K+
ATPase which pump 3 Na+ out- 2 K+ in
 Iodide then diffuses from the basal side to the apical side of the cell, where it
is transported into the colloid through the (pendrin) transporter
 Synthesis of thyroid hormone requires incorporation of iodine into tyrosines of
thyroglobulin
 The enzyme thyroid peroxidase (TPO ) catalyzes the oxidation, organification, and coupling of
iodotyrosine residues: (these steps in the lumen)
 Oxidation: TPO uses hydrogen peroxide(H2O2) to oxidize iodide (Iox).
 H2O2 is produced by DUOX and it is oxidizing agent.
 DUOX (apical enzyme ) has 2 activity:
- Oxidase to make H2O2 (rate limiting step)
- peroxidase
 Organification: TPO links tyrosine residues of thyroglobulin protein with
iodine.

 It generates monoiodotyrosine (MIT) and diiodotyrosine (DIT).

 Coupling reaction: TPO combines iodinated tyrosine residues to make triiodothyronine (T3) and
tetraiodothyronine (T4).

 MIT and DIT join to form T3, and two DIT molecules form T4.
 TSH and iodine on DUOX (highly regulated because it’s the rate limiting step)
 TSH →increase activity and expression of DUOX
 I → increase activitybu at the physiological level

 Wolff–Chaikoff effect :
When the I is very high in concentration it will inhibit DUOX this response is transient & immediate then cells adapt
to the high concentration so it stepover the inhibition & produce Thyroid hormone

Secretion of T3 and T4 into the bloodstream requires endocytosis of the thyroglobulin and
its proteolysis by lysosomal enzymes
 Storage:
Thyroid hormones are bound to thyroglobulin for storage in the follicular lumen.
 Release:

 Thyrocytes uptake iodinated thyroglobulin via endocytosis or macropinocytosis

1. Lysosome fuse with the endosome containing iodinated
thyroglobulin
2. Proteolytic enzymes in the endolysosome cleave thyroglobulin
into MIT, DIT, T3, and T4.
3. T3 and T4 (predominant) are released into the fenestrated
capillaries via MCT8 transporter.
 4. Deiodinase and halogenase enzymes remove iodine molecules from iodothronines and
iodotyrosines
5. Iodine can be recycled for the synthesis of thyroid hormone again.

 MCT8 is also important for uptake of T3 & T4 from blood into cells in different tissues

Thyroid hormones are lipophilic

 They circulate bound to the transport proteins
 Over 99% of T4 and T3 circulate in the blood bound to proteins.
 Very small fraction of T3 and T4 transport free in blood (0.3-0.03%) which is the active
one (it produce the biological effect
 Thyroxine-binding globulin (TBG) has the highst affinity to T3&T4
 Other two with less affinity for T3 and T4 (10-20% of thyroid hormone bound to them)
 Transthyretin (TTR); also known as thyroxine-binding prealbumin, TBPA),
 Albumin.

 TBG binds about 75% of both T4 and T3.

Deiodinases regulate thyroid hormone signaling at the cellular level by activating or

suppressing local T3 production
 T4 (prohormone)is the storage form and released when needed,
 T4 has longer half life 5-7 days (T3 →1-2 days)
 Why T3 is the active form and T4 don’t produce effect?
 Because T3 has higher affinity to the receptor (binding with the intranuclear
receptor)

 The Receptor activity vary weather it is bound to ligand or not .

 binding with T3→regulate expression of genes & regulate different processes inside the cell
 if the cell is done with thyroid hormones it will convert T3 & T4 into inactive form
 T4 is converted to the active thyroid hormone T3 by two deiodinases, D1 and D2, by outer-ring
deiodination.
 Both D1 and D2 catalyze the outer-ring deiodination (activation) of T4 to T3.

 Both T4 and T3 are also inactivated by inner-ring deiodination by D1 and D3

 Inner-ring deiodination by D3 leads to inactivation of T3 to 3,3′-di-iodothyronine, and of T4 to
rT3

 Activation and deactivation occur by deiodinase

 D1 & D3 →on inner ring ,,,,,,,,,,,,D2& D1 →outer ring
 Thyroid hormone is controlled by the hypothalamic-pituitary-thyroid axis
 The hypothalamus releases thyrotropin-releasing hormone (TRH) into the hypothalamic-
hypophyseal portal system to the anterior pituitary gland.
 TRH stimulates thyrotropin cells in the anterior pituitary to the release of thyroid-stimulating
hormone (TSH).
 T4 and T3 has negative feedback effects of on the pituitary(mainly) and hypothalamus.
 TRH binds to the TRH receptors on the anterior pituitary gland, causing a signal cascade
mediated by a G-protein coupled receptor; guanylyl cyclase & PLC pathway mainly
 TSH is released into the blood and binds to the thyroid-releasing hormone receptor (TSH-R)
on the basolateral aspect of the thyroid follicular cell to control the synthesis & secretion of
thyroid hormone.
 The TSH-R is a Gs-protein coupled receptor, and its activation leads to the activation of
adenylyl cyclase and intracellular levels of cAMP

Thyrotropin-releasing stimulates the synthesis and secretion of thyroid-hormone-

stimulating hormone (TSH)
TRH binds to the TRH receptors on the anterior pituitary gland, causing a signal
cascade mediated by a G-protein coupled receptor.

 GPCR either work on AC or GC

 The main pathway here is the Gc →PLC
 PLC convert PIP2 into DAG &IP3
 That will lead to ca+ flux and activation of different kinases
 Kinases act as Transcription factor to regulate gene expression
 TSH gene encode a heterodimeric protein (two different subunit):
 Alpha →shared with other hormones like FSH, LH
 Beta→ specific for FSH TSH

 Regulation of synthesis and excretion of TSH mainly on beta

 Binding of TRH to its receptor induce expression of beta subunit of TSH →alpha and beta dimerize →producing
functional form of TSH
 TSH stimulates the synthesis and secretion of T3 and T4
 TSH binds its TSH receptor on the basal membrane of thyroid epithelial cells.
 TSH binds G protein-coupled membrane receptors in the thyroid gland and activates adenylate
cyclase(AC mainly) with the generation of cAMP.
 cAMP activates protein kinase A which initiates a cascade of protein phosphorylations that results
in the secretion of thyroid hormone.
 Genes encoding protiens that are needed for the synthesis of thyroid hormone and need to be
upregulated:
 NIS
 TPO
 DUOX
 Tg (synthesis, packaging and sent to the lumen)

Thyroid hormones bind its intranuclear receptor to regulate gene expression

 TR affinity for T3 is 10–15 times greater than for T4.
 Thyroid H receptor are like steroid receptor (intracellular; inside the nucleus )
 Steroid and thyroid hormone are lipophilic with similar MOA but the differ in
their structure
 Thyroid hormone need transpotor for upptake by the cells (MTC8)
 T4 is designated as an inactive prohormone from which is derived the active
hormone, T3.
 T3 bind with the nuclear R to induce an effect
 Then it act like supressor or activator to certain genes (bind to DNA to control gene expression)
 And depending on the gene and the cell the effect can be repression or activation
 Other forms of thyroid receptor:
 Mitochondrial (regulate oxidation)
 Integral in plasma membrane

Thyroid hormones bind its intranuclear receptor to regulate gene expression

 the main pathway is the nuclear R
 The R is bound to DNA response element (TRE) to control
downstream gene
 If the R is free it recruit corepressor →modify chromatin structure to
repress the gene expression
 Once T3 is inside the cell → bind to R →replace corepressor with
coactivator & activate gene expression
 TR usually work with retinoic XR (RXR) for gene expression regulation
 Mainly :
 Receptor bind to T3 →activation
 absence of T3 →repressor

 But some times the opposite happen if the gene are downregulated by T3 (repressor if the R is bound to T3)

Thyroid receptor has four isoforms

 TRα1and TRα2 are predominantly expressed in brain, skeletal and cardiac
muscle, kidney, lungs, liver, and brown fat
 TRβ2 isoform is predominantly expressed in the anterior pituitary gland,
hypothalamus, retina, and the cochlea (feed back inhibition to decrease TRH TSH
and thus T3 and T4
 All have T3 binding site except TRα2
 TR alpha →during fetal life and early development
 TR beta→during adulthood


Thyroid hormones affect several tissues and influence major processes

 Metabolic Actions:

 Calorigenic effect
 Accelerated glucose oxidation is reflected in an increased basal metabolic rate (BMR) as measured by an
increased rate of oxygen consumption.
 Stimulate carbohydrate metabolism
 It can induce lipolysis or lipid synthesis- depends on the metabolic status
 Anabolic (synthesis) protein metabolism-under physiological TH levels, but higher than normal T3
→breackdown of protien
 Permissive effect on catecholamine.
 Thermogenic action

 T3 but not T4 stimulates the production of a unique mitochondrial protein known as uncoupling
protein 1 (UCP-1) (increase production of ATP especially in brown lipid)
 Growth and development: thyroid hormone also helps with neural maturation.(fetal, early life, and
eventually adulthood)
 Thyroid hormones affect several tissues and influence major processes
 Heart: thyroid hormones have a permissive effect on catecholamines. It increases the expression of beta-
receptors to increase heart rate,, cardiac output, and contractility
 Lung: thyroid hormones stimulate the respiratory centers and lead to increased oxygenation because of
increased perfusion.
 Nervous system: Hyperthyroidism can lead to hyperexcitability and irritability. Hypothyroidism can cause
impaired memory, slowed speech, and sleepiness.
 Reproductive system: Hypothyroidism is commonly associated with infertility. (deficient gonadogenisis)

Done by: Sura alnajjar