1. Thyroid Gland Parts I and II
The Gentlemen's Guide
Like a Sir

2. Learning Objectives
Identify the steps in the biosynthesis, storage, and secretion of tri-iodothyronine (T3) and thyroxine (T4) and their regulation.
Describe the absorption, uptake, distribution, and excretion of iodide.
Explain the importance of thyroid hormone binding in blood on free and total thyroid hormone levels.
Understand the significance of the conversion of T4 to T3 and reverse T3 (rT3) in extra-thyroidal tissues.
Describe the physiologic effects and mechanisms of action of thyroid hormones.
Explain what conditions can cause an enlargement of the thyroid gland.
Understand the causes and consequences of
over secretion of thyroid hormones
under secretion of thyroid hormones

3. Location of Thyroid Gland
canine
Located in the neck over the first part of the trachea
Humans-”butterfly” –two lateral lobes connect by an isthmus
Nodules on thyroid=parathyroid glands (2 superior/2 inferior)
Parafollicular (C cells)—major source for hormone calcitonin
Structural effects on the thyroid can also have an effect on the parathyroid cells.
Colloid contains thyroglobulin which is the basis for the formation of thyroid hormones.
Calcitonin is produced by parafollicular cells in the thyroid.

4. Chemistry of Thyroid Hormone
Thyroid hormones are derivatives of the amino acid tyrosine covalently bound to iodine
Iodine bound at either 3 or 4 positions on two linked tyrosines
Other iodinated molecules can be generated (example: rT3 but are inactive)
T3 and T4 are poorly soluble in water (blood) so are carried bound to proteins
Principle binding protein is thyroid-binding globulin
Other important carrier proteins are albumin and transthyrein
Carriers allow a stable pool of hormone in blood that can release free hormone to tissues (sites of action)
Tyrosine is taken up using a Na+ symporter.
T3 and T4 not soluble – bound to thyroid-binding globulin (major), albumin and transthyrein.

5. Steps in Biosynthesis, Storage and Secretion of T3/T4
Raw materials:
1: Tyrosine-sourced from thyroglobulin in colloid
thyroglobulin secreted by thyroid epithelial cells
2: Iodide (I-)-uptake from blood by thyroid
epithelial cells (outer plasma membrane has a
Na+/I- symporter) once in cell transported to
colloid with thryoglobulin
Tyrosine in thyroglobulin
Iodine from Na+/I- symporter in thyroid epithelial cells.

6. Steps in Biosynthesis, Storage and Secretion of T3/T4
1. Thyroid peroxidase adds the (iodide) I- to tyrosines on thyroglobulin (organifaction of
iodide).
2. Synthesis of thyroxine (T4) or tri-iodotyrosine (T3) from two iodotryosines
Remember that the hormone is still tied to the thyroglobulin ---needs to be liberated into
the circulation as needed
Synthesis in the thyroid gland in the follicle colloid. Specifically the iodination.
Thyroglobulin-bound hormone are cleaved in an Endocytic vesicle in the thyroid follicular cell and are exocytosed as free T3 or T4

7. Thyroid Hormone Cycle

8. Steps in Biosynthesis, Storage and Secretion of T3/T4
Release of T3/T4 from colloid:
Thyroid epithelial cells ingest iodinated thyroglobulin from apical surface (endocytosis)
Colloid filled endosomes fuse with lysosomes that contain hydrolytic enzymes that digest the iodinated thyroglobulin
Release of active thyroid hormone
Free thyroid hormone diffuses across the baso-lateral epithelial membrane into ECF (blood)
Free thyroid hormone quickly bound to carrier proteins to be transported and release at target cells.

9. Transport of Thyroid Hormone
T3/T4 are highly bound to plasma protein carriers
Major carrier is thyroxine–binding globulin (TBG)
Secondary carriers are albumin and thyroxine-binding prealbumin
Approximately 99.9% of T4 bound and <0.1% is free hormone
[T3]free in plasma <1% (more active than T4)
Because of tight binding to plasma proteins has long half life
(T4=7 days)
Free hormone is what is captured by target cells and
exerts its biological effect before degradation

10. Transport of T4/T3 to peripheral tissues
Increases (pregnancy) and Decreases (liver diseases) in circulating plasma
TBG levels change the amount of TBG bound hormone
However, They only transiently effect the amount of biologically active FREE hormone because the negative feedback of free hormone on TSH levels
Remember: TSH stimulates release of free hormone
Example: In pregnancy, a fall in T3 levels, causes a compensatory increase in TSH levels that in turn will increase production of free hormone in the circulation. Increased thyroid hormone shuts off TSH
But total [free hormone = T3] may be higher
TRF target is anterior pituitary.
TSH target is the thyroid.

11. Thyroxine (T4) conversion to T3
T4 is the dominant secreted/circulated form from the thyroid
However:
Most of the T4 secreted by thyroid is metabolized to T3
De-iodinated at the 5’ or 5 position in peripheral tissues to either T3 or rT3 (inactive)
Since T4 is primarily converted to T3 that has a higher affinity for thyroid hormone receptors ---sometimes T4 is considered a prohormone for T3
Ratio of T4 to T3 is 5:1 in circulation
Potency of T4 to T3 is 1:10 (affinity)
T4 is converted to T3 by peripheral peroxidase
T3 has much higher affinity for peripheral receptor than T4 does.

12. Mechanism of Action and of T3/T4
Receptors for thyroid hormone action are INTRACELLUAR DNA-BINDING PROTEINS
Function as hormone responsive transcription (HRE) factors
Mode of action is similar to steroid hormones
T3 binds to short repetitive sequences of DNA called Thyroid Response Elements (TRE)
TRE DNA sequence= AGGTCA
Can be arranged as direct repeats, pallindromes or inverted repeats.
Can be monomer (AGGTCA) homodimer (AGGTCANNNNAGGTCA) or a heterodimer
(T3+ another protein in complex) with the retinoic acid receptor (RXR) another member
of the nuclear receptor subfamily
Heterodimer is the high affinity form and thought to be the major functional entity
Effects of T3 are mediated by gene transcription. De novo synthesis is major method of action.

13. Mechanism of Action and of T3/T4
Thyroid hormone receptors bind to the TRE DNA with and without T3
Biological effects of T3 bound vs. T3 unbound receptor are dramatically different
Generally:
1: Binding of receptor w/o T3 to DNA transcriptional repression
results in compacted “turned off” chromatin
2: Binding of T3+receptor complex transcriptional activation
complex recruits different co-activators proteins – conformational changesleads to gene activation
HDA=histone deactelyase
HAT=histone transacetylase
Unbound receptor acts as a repressor.
T3 bound receptor acts as gene transcription.

14. Thyroid Hormone Receptors Structure
Mammalian Thyroid Receptors are encoded by two genes α and β
Three Functional Domains:
Transactivation
amino terminus---interacts with other transcription factors
2. DNA Binding
Recognition of HRE for binding
3. Ligand-Binding and dimerization
T3 lignad binding at carboxy terminus
Primary gene transcript for both forms can be alternately spliced to form
multiple α and β isoforms.
Binding domain is associated with change of functions which will affect receptor function.

15. Thyroid Receptors
The different isoforms of thyroid receptor have patterns of expression that differ
by tissue and developmental stage.
Examples:
α1, α2 and β1 isoforms are expressed in virtually all tissues
β2 found predominantly in hypothalamus, anterior pituitary and developing ear
α1 first to be expressed in fetus and β1, β2 up-regulated in developing brain after birth
They activate several genes known to important in brain development such as myelin basic protein

16. Regulation of Thyroid Hormone Secretion
Hypothalamic-pituitary-thyroid axis
Control of thyroid hormone secretion is a NEGATIVE FEEDBACK LOOP
Binding of TSH on thyroid epithelia enhances all the processes for hormone synthesis of
1. Iodide transporter
2. Thyroid peroxidase
3. Thyroglobulin
Magnitude of the TSH signal sets the rate for endocytosis of colloid
 TSH Faster rates of colloid synthesis hence more
hormone in circulation
 TSH Decrease in colloid synthesis hence less
COLD Exposure can increase TRH release  enhanced thyroid hormone release

17. Degradation of Thyroid Hormone
These steps occur in peripheral tissues:
Deiodination and decarboxylation
Glucuronidation/Sulfonation in liver
Excretion into bile ducts
Excretion of glucuronide conjugate in urine

18. Thyroid Hormone and Cellular Metabolic Rate
Many of the effects of T3 are secondary to increased BMR
 sweating and thermogenesis – heat elimination from skin
 Rate and depth of breathing – need for O2
 Cardiac output – from increases in SV and HR and changes in contractile force
Improve memory and learning capacity
Most tissues
Increase in O2 consumption
Increase in heat production
Mitochondria increase in size and number
Key respiratory enzymes increase

19. Physiological Effects of Thyroid Hormone
Metabolism:
Thyroid hormone stimulates most tissues of the body resulting in an increase of basal metabolic rate
This involves:
Increases body temperature
O2 consumption and ATP hydrolysis
Stimulates fat mobilization leading to increases of [FA] in plasma
FA oxidation is also increased
Increases in carbohydrate metabolism
increased insulin dependent glucose entry into cells, gluconeogenesis ,glycogenolysis
Growth:
Thyroid hormone synergistically combines with growth hormone to enhance grow processes

20. Physiological Effects of Thyroid Hormone
Development:
Critical for development of fetal and neonatal brain
Cardiovascular:
Promotes vasodilation ---increased vascular flow in organs
Increases cardiac output, cardiac rate, and cardiac contractility
CNS:
Decreased hormone sluggish mental activity
Increased hormone related to anxiety
Reproduction:
Low hormone levels frequently associated with infertility (female) (male?)

21. Thyroid and Bone Bone cells have receptors for Thyroid hormone:
Necessary for growth and maturation of the skeleton
If thyroid hormone levels are too high
Osteoporosis and bone loss

22. Thyroid and the Heart: T3-Responsive Genes for Important Cardiac Proteins
Positive regulation-increased gene expression
Sarcoplasmic reticulum calcium adenosine triphosphatase
Myosin heavy chain α
β1-Adrenergic receptors
Guanine-nucleotide-regulatory proteins
Sodium/potassium adenosine triphosphatase
Voltage-gated potassium channels
Negative regulation-decreased gene expression
T3 nuclear receptor α1
Myosin heavy chain β
Phospholamban
Sodium/calcium exchanger
Adenylyl cyclase types V and VI

23. Thyroid Function in Pregnancy and Fetal Development
Maternal Thyroid Function in Pregnancy:
Normal changes in thyroid function during pregnancy include:
1a) 2-fold increase T4 (thyroxine) binding globulin (TBG) stimulated by increases in estrogen
1b) Increased levels of TBG decrease free T4 therefore more TSH is made, in turn, increasing T3 and T4
1c) Increased amounts of thyroid hormone balance reached at 20 weeks and maintained until parturition
2. Increased demand for iodine—significant increase in pregnancy clearance of I- by kidney and siphoning of I by fetus from maternal circulation
3. Thyroid stimulation by hCG—TSH and hCG similar enough that hCG can increase mimics TSH at gland receptor stimulates T3 release while TSH maybe suppressed (graph)
Some women may develop transient hyperthryoidosis in pregnancy or if a woman has subclinical hypothryoidism the
demand of fetus can precipitate hypothyroidism.

24. Thyroid and Fetal Brain
Thyroid receptors present in brain tissue before fetus is synthesizing its own hormone.
During fetal brain development thyroid hormone assists in activation of genes (HRE) involved with terminal stages of brain differentiation
1. Synaptogenesis
2. Growth of dendrites and axons
3. Myelination
4. Neuronal migration

25. Thyroid Hormone Resistance
Mutations in β receptor gene that abolish ligand-binding
In most families transmitted as dominant trait
Clinically:
Hypothyroidism with goiter
Elevated serum [T3] and [thyroxine] and normal or elevated serum [TSH], significant number of pediatric patients show attention deficit disorder.
Activity is decreased because there is no receptor binding.

26. Graves Disease Most common form of hyperthyroidism
Antibodies Thyroid stimulating immunoglobulins (TSIs) form against the TSH receptor of the thyroid gland
TSI’s bind to TSH receptor and mimic action of TSH
Results in Goiter and increases thyroid hormone and decreases in TSH because of negative feedback due to increased thyroid hormone in plasma

27. Predicted Changes in Graves
Increased metabolic rate
Heat intolerance and sweating
Increased appetite but weight loss
Palpitations and tachycardia
Nervousness and emotional swings
Muscle weakness
Tiredness but inability to sleep
Many patient’s develop protruding eyeballs (exopthalmos)
Degenerative changes in extraocular muscles resulting from autoimmune reaction

28. Hypothyroidism Hashimoto’s Thyroiditis
Autoimmune destruction of gland
Also called Autoimmune Thyroiditis
Predicted changes
1) Decreased metabolic rate
2) Cold intolerance and decreased sweating
3) Weight gain w/o increased appetite
4) Bradycardia
5) Slowness of speech, thought and movement
6) Lethargy and sleepiness
Accummulation of mucoplysacchrides in interstial spaces----”puffiness” of skin myxedema
Decreased Thyroid hormone  increased TSH levels (may lead to compensatory goiter)

29. Thyroid Deficiency In Fetus and Neonate
Maternal
Fetus has two potential sources of thyroid hormone
Fetus begins to make T3/T4 at approximately 12 weeks gestation
Substantial transfer of hormone across the placenta
placenta has a de-iodinases that converts T4 to T3
Three forms of hypothyroidism in Pregnancy:
Isolated Fetal Hypothyroidism
Isolated Maternal Hypothyroidism
Iodide Deficiency (combined maternal and fetal hypothyroidism)
Fetal

30. Cretinism
Severe hypothyroidism resulting stunted growth and mental retardation
Athyrotic cretinsim
thyroid aplasia or Iodine deficiency in utero
Endemic cretinism
iodine deficiency
Spasticity, deaf-mutism, motor dysfunction
Symptoms
Puffy face, short stature, protruding abdomen and swollen tongue, slow reflexes

31. Iodide Deficiency Combined maternal-fetal hypothyroidism
Most common cause of preventable mental retardation world-wide
Results in cretinism, mental retardation,deaf mutism and spasticity
Supplement with iodine in 1st and 2nd trimester (later will not prevent defects)
Endemic Goiter
high plasma TSH stimulates gland

32. Hyperthyroidism in Pregnancy Gestational Hyperthyroidism
Increased risk for:
Preeclampsia
Premature labor
Fetal or perinatal death
Low birth weight
May be caused by Grave’s Disease
Auto-antibodies against TSH receptor

33. Fetal Hypothyroidism Sporadic Congenital Hypothyroidism
Fetal gland doesn’t produce enough hormone
Normal at birth (maternal compensation)
Needs rapid diagnosis shortly after birth or risk child having permanent mental and growth retardation

34. Maternal Hypothyroidism
Female hypothyroidism frequently associated with infertility
If pregnancy does occur there is an increased risk of fetal death and gestational hypertension
Subclinical maternal hypothyroidism
Diagnosed retrospectively
Auto-antibodies to thyroid that can cross placenta
Children with lower IQ scores

35. Hamburger Thyrotoxicosis Rare Case
A 61 yr. old woman in Canada presented to physicians with intermittent hyperthyroidism that would resolve on its own over 2-3 months. They saw rapid weight loss, increased sweating and palpitations, tremor in her hands and a heart rate of 112 beats/minute.
Diagnosis was confirmed by and elevated free T4 (46 compared to 9-23) and a very low TSH. Within 2 months her symptoms disappeared on their own and her T4 returned to normal range. She did not have thyroid antibodies. The woman had 5 such episodes over an 11 year period. The physicians were left puzzled.
Resolution:
Patient indicated she did not take herbal supplements or thyroid supplements
Additional questioning about her dietary habits revealed the cause.
She lived on a farm and every year a cow was slaughtered and the butcher packaged the meat for use. It was discovered that the butcher did not know that “gullet trimming” was prohibited. Hence, muscles from the larynx and thyroid were trimmed and used to make hamburger patties. The woman consumed these (her husband did not) patties and had exposure to cow thyroid gland.
—similar cases seen in Minnesota, South Dakota and Iowa.

36. Key Concepts How is thyroid hormone regulated ?
Why is mode of action like a steroid hormone?
Over secretion of thyroid hormone has what consequences?
Under secretion of thyroid hormone has what consequences?
What is the active form of the hormone?
How does hormone reach tissues?
What are the actions at tissue level?
What amino acid forms the backbone of thyroid hormone?

37. Sample Board Question
Blood levels of ________would be decreased in Grave’s Disease.
Tri-iodothyronine (T3)
Thyroxine (T4)
Diiodothyrosiine (DIT)
Thyroid Stimulating Hormone (TSH)
Iodide (I-)
Answer
Recognize that Grave’s Disease is a form of hyperthyroidism. What’s happening? auto-antibodies (TSIs) are stimulating large increases in hormone by acting at the TSH receptors. Increased [hormone] is circulating and accessed in the CNS (anterior pituitary and hypothalamus) where TSH is decreased by the negative feedback loop (review regulation of thyroid).