Multiple Choice Questions (MCQs) on Thyroid gland hormones, thyroid disorders and Function test: Biochemistry

Thyroid Hormone Synthesis, Disorder, and Function Test- Multiple Choice Questions

                               

1) Which of the following amino acid is the precursor for the synthesis of thyroid hormones?
a) Tryptophan
b) Tyrosine
c) Alanine
d) Proline

2) Which of the following form is the active form of thyroid hormone?
a) T3
b) T4
c) rT3
d) None of the above

3) Which of the following proteins is the precursor for the thyroid hormone, and also a marker of thyroidal cancer?
a) Thyroid albumin
b) Thyroglobulin
c) Thyroid binding globulin
d) All of the above

4) The majority of the thyroid hormones in the blood are bound to proteins.

 Which of the following are not the thyroid hormone-binding proteins in the plasma?
a) Albumin
b) Thyroglobulin
c) Thyroid binding globulin
d) None of the above

5) Which of the following is the transporter responsible for the transport of iodine into the thyroid cells against the concentration gradient?
a) Na+ I- symport
b) K+ I- symport
c) I-/Cl- antiport
d) None of the above

6) Thyroid peroxidase is an enzyme responsible for thyroid hormone synthesis. This enzyme catalyzes the following reaction, except?
a) Conversion of iodide to iodine-free radical
b) Incorporation of iodine into a tyrosine residue of thyroglobulin
c) Condensation of monoiodo-tyrosine and diiodotyrosine
d) Cleavage and release of thyroid hormones

7) Which of the following chemical drug also reduces the uptake of iodine into the thyroid cells?
a) Lithium
b) Methimazole
c) Thiourea
d) All of the above

8) The thyroid hormone synthesis is regulated by thyroid-stimulating hormone (TSH) synthesized by the anterior pituitary gland. 

The increased thyroid hormone synthesis is mediated by:
a) increased cGMP production in follicular cells
b) increased cAMP production in follicular cells
c) increased Ca+2 ions in the follicular cells
d) increased diacylglycerol production in follicular cells

9) Which of the following cellular processes increase in response to thyroid-stimulating hormones?
a) Uptake of iodide into the follicular cells
b) Synthesis of thyroglobulin proteins
c) Incorporation of iodide ions into tyrosine molecules of thyroglobulin
d) All of the above

10) The deiodinases are responsible for the removal of iodine from T4 molecules to produce T3 & rT3. Which type of enzyme catalyzes the conversion of rT3?
a) Type I
b) Type II
c) Type III
d) None of the above

11) In pregnancy, TBG levels increase, leading to an increase in total thyroid hormone, but the normal levels of free hormone. This is known as..................................
a) Euthyroid state
b) Hyperthyroid state
c) Hypothyroid state
d) Subclinical hypothyroid state

12) In peripheral tissues, T4 is converted to T3 by which of the following enzyme?
a) 5'-iodinase
b) 3'-iodinase
c) 5-iodinase
d) 3-iodinase

13) The hypothalamic-pituitary and thyroid axis regulates the synthesis and release of thyroid hormones. 

Which of the following receptors are down-regulated by excess T3?
a) TSH receptor in the thyroid gland
b) T3 receptor in hypothalamus
c) TRH receptor in the anterior pituitary
d) All of the above

14)Patients with Graves disease have high circulating levels of thyroid hormones. Graves disease is characterized by the following, except?
a) Excess release of TSH from the anterior pituitary
b) Hyperactivation of TSH receptor in thyroid glands
c) Presence of TSH receptor antibodies causing receptor activation
d) Decreased TSH levels

15) Which of the following is not the metabolic function of thyroid hormones?
a) Glycogenolysis
b) Gluconeogenesis
c) Glucose oxidation
d) Glycogenesis

16) The metabolic function of thyroid hormones include
a) Decreased oxygen consumption
b) Increased lipogenesis
c) Increased Lipolysis
d) Protein Anabolism

17) A 50 years old man with hyperthyroidism is treated with propylthiouracil. The drug reduces the synthesis of thyroid hormones because it inhibits the oxidation of
a) Triiodothyronine
b) Thyroxine
c) Diiodotyrosine
d) Iodide

18) Which of the following is the screening test for the diagnosis of thyroid disease?
a) Thyroid Stimulating hormone
b) Total T3
c) Total T4
d) Free T3

19) The subclinical hyperthyroidism is characterized by............................................................................
a) Increased TSH and normal free T3 & free T4 levels in the blood
b) Increased TSH and decreased free T3 & free T4 levels in the blood
c) Decreased TSH and normal free T3 & free T4 levels in the blood
d) Decreased TSH and increased free T3 and free T4 levels in the blood

20) A patient presents with tachycardia and heat intolerance and is suspected of Graves disease. 

Which of the following is not a characteristic feature of  Grave disease?
a) Increased total and free T4
b) Suppressed plasma TSH
c) Exophthalmos
d) Decreased thyroid radioactive iodine uptake

Answers

1-b) Tyrosine

Thyroid hormones, such as thyroxine (T4) and triiodothyronine (T3), are derived from the amino acid tyrosine. In the thyroid gland, tyrosine is used as a substrate to synthesize these hormones through a series of enzymatic reactions that involve the incorporation of iodine atoms into the tyrosine molecule. The addition of iodine to tyrosine results in the formation of T3 and T4, which are crucial for regulating various metabolic processes in the body.

2-a) T3

While both T3 and T4 (Triiodothyronine and Thyroxine) are produced by the thyroid gland, T3 is the more biologically active form of thyroid hormone. T4 is produced in larger quantities but is generally considered a prohormone, as it is converted into T3 in various tissues throughout the body. T3 is the form of thyroid hormone that directly affects the regulation of metabolic processes and has a more potent effect on target cells and tissues.

Reverse T3 (rT3) is an inactive form of thyroid hormone and does not have the same biological activity as T3. 

3-b) Thyroglobulin

Thyroglobulin is a large glycoprotein produced by the thyroid gland. It serves as a storage form of thyroid hormones and is involved in their synthesis. In thyroid cancer, the measurement of thyroglobulin levels can be used as a marker to assess the presence of residual thyroid tissue or recurrent thyroid cancer after treatment, particularly following thyroid surgery or radioactive iodine therapy. Elevated levels of thyroglobulin in the blood can indicate the presence of thyroid cancer or the persistence of thyroid tissue in the body, making it a valuable marker for monitoring thyroid cancer patients.

4-b) Thyroglobulin

The majority of thyroid hormones in the blood are bound to proteins, primarily two specific proteins: thyroxine-binding globulin (TBG), and to a lesser extent, thyroxine-binding prealbumin (TBPA) and albumin.

Thyroxine (T4) and triiodothyronine (T3), are lipophilic (fat-soluble) molecules, and they are not very water-soluble. To be transported in the bloodstream, they bind to these transport proteins. This binding helps to maintain a stable and sustained level of thyroid hormones in the blood and also protects them from being rapidly metabolized or excreted. Only a small fraction of thyroid hormones in the blood is in their free, unbound form, and it's the free hormones that are biologically active and available to affect target tissues.

5-a) Na+ I- symport

The transporter responsible for the transport of iodine into thyroid cells against the concentration gradient is called the sodium-iodide symporter (NIS). The sodium-iodide symporter is a membrane protein found in the thyroid follicular cells and is crucial for the uptake of iodide ions from the bloodstream into the thyroid gland.

This active transport process allows the thyroid gland to concentrate iodine from the blood, which is an essential step in the synthesis of thyroid hormones. The iodine is taken up against its concentration gradient, and this accumulation of iodine within the thyroid cells is a critical initial step in the production of thyroxine (T4) and triiodothyronine (T3), the two main thyroid hormones.

6-d) Cleavage and release of thyroid hormones

Thyroid peroxidase (TPO) is an enzyme involved in the synthesis of thyroid hormones, and it plays a key role in the following reactions in the thyroid gland:

Iodination of Tyrosine: TPO catalyzes the incorporation of iodine atoms onto the amino acid tyrosine within thyroglobulin, a protein produced by the thyroid gland. This iodination process is a crucial step in the formation of thyroid hormones.

Coupling of Iodotyrosines: TPO also catalyzes the coupling of two iodotyrosine molecules to form either thyroxine (T4) when two molecules are coupled or triiodothyronine (T3) when only one molecule is coupled.

7-a) Lithium

Lithium is a chemical drug that not only inhibits thyroid hormone synthesis but also reduces the uptake of iodine into thyroid cells. It is used in the treatment of certain thyroid disorders, particularly in cases of hyperthyroidism (overactive thyroid). Lithium interferes with the release of thyroid hormones and decreases the uptake of iodine by the thyroid gland, thus reducing the production of thyroid hormones. This can help manage hyperthyroidism by lowering thyroid hormone levels.

8-b) increased cAMP production in follicular cells
9-d) All of the above
10-c) Type III
11-a) Euthyroid state
12-a) 5'-iodinase
13-c) TRH receptor in the anterior pituitary
14-a) Excess release of TSH from the anterior pituitary
15- d) Glycogenesis
16-c) Increased Lipolysis
17-d) Iodide
18)-a) Thyroid Stimulating hormone
19-c) Decreased TSH and normal free T3 & free T4 levels in the blood
20-d) Decreased thyroid radioactive iodine uptake