Download T-3/T-4

Triiodothyronine (T3) and
Thyroxine (T4)
Peter Schulz
What Are They?
Triiodothyronine (T3) and Thyroxine (T4) are the
two main hormones that are produced by the
thyroid gland
Thyroid gland→ located at the front of the neck
below Adam’s apple (larynx)
Butterfly-shaped and consists of two lobes→ on both
sides of trachea
Thyroid functions to regulate body’s metabolic rate,
promote growth and development
Also heart and digestive function, muscle control, brain
development, and bone maintenance
Thyroid is under control of the
pituitary gland and Thyroid
Stimulating Hormone (TSH)
What Are They?
Triiodothyronine (T3) is the active form of the hormone
Thyroxine (T4) is the inactive form of the hormone (prehormone) → must be
converted within its target cells into the active form → T3
T4 and T3 are secreted by the body in a ratio of 9:1
Thyroxine (T4) travels through blood attached to carrier proteins (99.96%) → it must
be free or unbound to enter target cells→ transformed into active form (T3)
Synthesis: occurs in the thyroid itself
To make both T3 and and T4→ need iodine in diet!
Synthesis:
→ tyrosine
Iodine is bound to tyrosine residues in thyroglobulin molecules→
forms mono-iodotyrosine (MIT) and di-iodotyrosine (DIT)
(MIT) + (DIT) = Triiodothyronine (T3)
(DIT) + (DIT) = Tetraiodothyronine (Thyroxine) (T4)
Potassium iodide
(1924)
Sidenote:
A goiter is an enlargement of the thyroid gland
Most commonly caused by lack of iodine in diet
Grave’s disease: (hyperthyroidism) → too much
thyroxine (T4) produced
Hashimoto’s disease: (hypothyroidism) → too little
thyroxine (T4) produced
Mechanism of Thyroid
Hormone
1) T4 carried to target cell bound to
its plasma carrier protein
(thyroxine-binding globulin or
TBG) → dissociates from carrier→
passes through membrane of target
cell
4
5
3
6
2)T4 converted to T3 in cytoplasm
(enzymatically)
3) T3 uses binding proteins to enter
nucleus
1
*lipophilic
hormone →
pass through
membrane
2
7
Mechanism of Thyroid
Hormone
4) Hormone receptor complex binds to
DNA
4
5
5) Binding to DNA stimulates synthesis
of new mRNA
3
6
6) Newly formed mRNA codes for
synthesis of new proteins
1
7) Proteins produce hormonal effects in
target cell
2
7
How are the Thyroid Hormone Receptors
Different From Normal Steroid Receptors?
In contrast to steroid hormone receptors, thyroid hormone receptors bind
DNA in the absence of hormone (T3)→ transcriptional repression
Hormone binding is associated with a conformational change in the thyroid
hormone receptor protein that causes it to function as a transcriptional
activator→ protein product synthesis causes desired effect
ORANGE receptor proteins would bind to DNA WITHOUT hormone and
cause transcriptional repression
This shows normal steroid
receptors, NOT thyroid
hormone receptors
Symptoms/Effects?
Listed below are other symptoms of too much
T3 and T4 in your body (hyperthyroidism):
The following is other symptoms of too little T3
and T4 in your body (hypothyroidism):
Anxiety
Trouble sleeping
Irritability or moodiness
Tiredness and fatigue
Nervousness, hyperactivity
Difficulty concentrating
Sweating or sensitivity to high temperatures
Dry skin and hair
Hand trembling (shaking)
Depression
Hair loss
Sensitivity to cold temperature
Missed or light menstrual periods
Frequent, heavy periods
Sources
http://www.endocrineweb.com/conditions/thyroid-nodules/thyroid-gland-controlsbodys-metabolism-how-it-works-symptoms-hyperthyroi
https://www.boundless.com/physiology/textbooks/boundless-anatomy-and-physiologytextbook/endocrine-system-16/the-thyroid-gland-156/formation-storage-and-release-ofthyroid-hormones-789-4783/
http://www.mayoclinic.org/diseases-conditions/goiter/symptoms-causes/dxc-20264595
http://arbl.cvmbs.colostate.edu/hbooks/pathphys/endocrine/thyroid/receptors.html