Synthesis Of Thyroid Hormone

SYNTHESIS OF THYROID HORMONE

Q. 1 Wolf-Chaikoff effect is seen due to? 
 A Inhibition of cAMP response of TSH 
 B Inhibition of proteolysis of Thyroglobulin 
 C Inhibition of organic binding of iodide
 D

Inhibition of iodide trapping mechanism

Q. 1 Wolf-Chaikoff effect is seen due to? 
 A Inhibition of cAMP response of TSH 
 B Inhibition of proteolysis of Thyroglobulin 
 C Inhibition of organic binding of iodide
 D

Inhibition of iodide trapping mechanism

Ans. C

Explanation:

Inhibition of organic binding of iodide

REF: Goodman and Gilman’s 9th edition page 1402, Guyton 11th edition page 939-940, http:// en.wikipedia.org/wiki/Wolff%E2%80%93Chaikoff_effect

The Wolff-Chaikoff effect is a reduction in thyroid hormone levels caused by ingestion of a large amount of iodine. It is an autoregulatory phenomenon that inhibits organification (oxidation of iodide) in the thyroid gland, the formation of thyroid hormones inside the thyroid follicle, and the release of thyroid hormones into the bloodstream.

  • The Wolff-Chaikoff effect lasts several days (around 10 days), after which it is followed by an “escape phenomenon” which is described by resumption of normal organification of iodine and normal thyroid peroxidase function.
  • The Wolff-Chaikoff effect also explains the hypothyroidism produced in some patients by several iodine-containing drugs, including amiodarone.
  • The Wolff-Chaikoff effect can be used as a treatment principle against hyperthyroidism (especially thyroid storm) by infusion of a large amount of iodine to suppress the thyroid gland. Iodide was used to treat hyperthyroidism before antithyroid drugs such as propyithiouracil and methimazole were developed.

Q. 2

The thyroid hormone thyroxine (T4) is derived from?

 A

Threonine

 B

Tyrosine

 C

Thiamine

 D

Tryptophan

Q. 2

The thyroid hormone thyroxine (T4) is derived from?

 A

Threonine

 B

Tyrosine

 C

Thiamine

 D

Tryptophan

Ans. B

Explanation:

Thyroxine is a derivative of tyrosine. It is formed by the iodination and joining of peptide-linked tyrosyl residues of thyroglobulin. Proteolysis of thyroglobulin yields thyroxine. Thyroxine is also called tetraiodothyronine, or T4, because of the four iodine atoms of the thyroid hormone. 

Ref:Weil P. (2011). Chapter 41. The Diversity of the Endocrine System. In D.A. Bender, K.M. Botham, P.A. Weil, P.J. Kennelly, R.K. Murray, V.W. Rodwell (Eds),Harper’s Illustrated Biochemistry, 29e.

Q. 3

Which of the following hormone is synthesized from the amino acid Tyrosine?

 A

Calcitriol

 B

Calcitonin

 C

Thyroxin

 D

Cortisol

Q. 3

Which of the following hormone is synthesized from the amino acid Tyrosine?

 A

Calcitriol

 B

Calcitonin

 C

Thyroxin

 D

Cortisol

Ans. C

Explanation:

The thyroid hormones triiodothyronine (T3) and thyroxine (T4) in the colloid of the thyroid also are derived from tyrosine.

In dopaminergic cells in the brain, tyrosine is converted to levodopa by the enzyme tyrosine hydroxylase (TH). Tyrosine is also the precursor to the pigment melanin.


Q. 4

During the metabolism of iodine in thyroid synthesis, Wolff-Chaikoff effect is seen due to:

 A

Inhibition of cAMP response of TSH

 B

Inhibition of proteolysis of Thyroglobulin

 C

Inhibition of organic binding of iodide

 D

Inhibition of iodide trapping mechanism

Q. 4

During the metabolism of iodine in thyroid synthesis, Wolff-Chaikoff effect is seen due to:

 A

Inhibition of cAMP response of TSH

 B

Inhibition of proteolysis of Thyroglobulin

 C

Inhibition of organic binding of iodide

 D

Inhibition of iodide trapping mechanism

Ans. C

Explanation:

Iodine metabolism within the thyroid can be regulated independently of TSH. This mechanism is important when plasma iodide levels are elevated (15–20-fold above normal) because this elevation inhibits the organic binding of iodine within the thyroid. This autoregulatory phenomenon consisting of inhibition of the organification of iodine by elevated circulating levels of iodide is known as the Wolff-Chaikoff effect. This effect lasts for a few days and is followed by the so-called escape phenomenon, at which point the organification of intra-thyroidal iodine resumes and the normal synthesis of T4 and T3 returns.

 
Ref: Molina P.E. (2013). Chapter 4. Thyroid Gland. In P.E. Molina (Ed), Endocrine Physiology, 4e.

Q. 5

Which of the following amino acids is involved in the synthesis of thyroxine ?

 A

Glycine

 B

Methionine

 C

Threonine

 D

Tyrosine

Q. 5

Which of the following amino acids is involved in the synthesis of thyroxine ?

 A

Glycine

 B

Methionine

 C

Threonine

 D

Tyrosine

Ans. D

Explanation:

Ans:D.)Tyrosine.

Specialized products from amino acids
Amino acid             Product
Glycine     :    Creatine, glutathione, heme, purines, conjugated bite acids
Tyrosine     :  Thyroxine, epinephrine, nor epinephrine, dopamine, melanin
Tryptophan  :  NAP, NADP, serotonin, melatonin
Methionine :   Creatine, epinephrine, poiyamines
Cysteine    :   Glutathione, taurine, co enzyme-A, active sulfate
Histidine   :   Histamine
Arginine   :   Creatine , nitric oxide
Lysine     :    Carnitine
Glutamate :  GABA, glutathione, gamma- carboxy glutamate
Glutamine :   Purines, pyrimidines, amino sugars
Aspartate  :   Purines, pyrimidines,
Serine      :    Phosphatidyt serine, sphingomyelins, choline
β-Alanine  :   Coenzyme-A


Q. 6

Wolf Chaikoff effect:

 A

Iodine induced hypothyroidism

 B

Iodine induced hyperthyroidism

 C

Thyrotoxicosis due to excessive amount of thyroid hormone ingestion

 D

Drug induced hyperthyroidism

Q. 6

Wolf Chaikoff effect:

 A

Iodine induced hypothyroidism

 B

Iodine induced hyperthyroidism

 C

Thyrotoxicosis due to excessive amount of thyroid hormone ingestion

 D

Drug induced hyperthyroidism

Ans. A

Explanation:

Answer is A (Iodine Induced Hypothyroidism)

 Repeat Iodine Induced Transient Suppression of Thyroid Gland is called Wolf Chaikoff Effect.


Q. 7

Amino acid required for formation of thyroxine‑

 A

Tryptophan

 B

Tyrosine

 C

Glutamine

 D

Cysteine

Q. 7

Amino acid required for formation of thyroxine‑

 A

Tryptophan

 B

Tyrosine

 C

Glutamine

 D

Cysteine

Ans. B

Explanation:

Ans. is ‘b i.e., Tyrosine

  • The term thyroid hormone denotes both thyroxine (7′4) and triodothyronine (T3).
  • Both T4 and T3 are iodine derivatives of thyronine.
  • Thyronine is a condensation product of two molecules of amino acid tyrosine.
  • Thyroxine is 3, 5, 3′, 5′ – tetraiodothyronine and T3 is 3, 5, 3 ‘-triiodothyronine.
  • Thyroid hormones are synthesized and stored in the thyroid follicles as apart of thyroglobulin molecule.


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