1. Triiodothyronine (T3) and Thyroxine (T4)
Peter Schulz

2. 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)

3. T4 and T3 are secreted by the body in a ratio of 9:1
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)

4. Synthesis: occurs in the thyroid itself
To make both T3 and and T4→ need iodine in diet!
Synthesis:
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)
→ tyrosine
Potassium iodide (1924) → → → →

5. 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

6. Mechanism of Thyroid Hormone
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
T4 converted to T3 in cytoplasm (enzymatically)
T3 uses binding proteins to enter nucleus 4 5 3 6
*lipophilic hormone → pass through membrane 1 7 2

7. Mechanism of Thyroid Hormone
4) Hormone receptor complex binds to DNA
5) Binding to DNA stimulates synthesis of new mRNA
6) Newly formed mRNA codes for synthesis of new proteins
7) Proteins produce hormonal effects in target cell 4 5 3 6 1 7 2

8. This shows normal steroid receptors, NOT thyroid hormone receptors
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

9. Symptoms/Effects?
Listed below are other symptoms of too much T3 and T4 in your body (hyperthyroidism):
Anxiety
Irritability or moodiness
Nervousness, hyperactivity
Sweating or sensitivity to high temperatures
Hand trembling (shaking)
Hair loss
Missed or light menstrual periods
The following is other symptoms of too little T3 and T4 in your body (hypothyroidism):
Trouble sleeping
Tiredness and fatigue
Difficulty concentrating
Dry skin and hair
Depression
Sensitivity to cold temperature
Frequent, heavy periods
Joint and muscle pain

10. Sources
bodys-metabolism-how-it-works-symptoms-hyperthyroi
textbook/endocrine-system-16/the-thyroid-gland-156/formation-storage-and-release-of- thyroid-hormones /