Thyroid Gland Questions in English

(A) During pregnancy,the metabolic rate of the mother increases significantly to support the developing fetus.
To meet these increased metabolic demands,the thyroid gland becomes more active,leading to an increased level of thyroxine in the maternal blood.
Therefore,Assertion $(A)$ is correct.
Reason $(R)$ is also correct because pregnancy involves various physiological and metabolic adaptations in the mother's body.
Since the increase in thyroxine is a direct consequence of the increased metabolic requirements during pregnancy,$(R)$ is the correct explanation of $(A)$.

(D) Thyroid hormones play a crucial role in the regulation of the basal metabolic rate $(BMR)$.
They are also involved in the maintenance of water and electrolyte balance.
Furthermore,they support the process of red blood cell $(RBC)$ formation (erythropoiesis).
However,the normal rhythm of the sleep-wake cycle is primarily regulated by melatonin (secreted by the pineal gland),and the development of the immune system is not a primary function of thyroid hormones.
Therefore,$C$ and $D$ are not under the control of thyroid hormones.

(B) The thyroid gland is indeed composed of follicles and stromal tissue. The follicles are lined by follicular cells which synthesize the hormones $T_3$ (triiodothyronine) and $T_4$ (thyroxine).
Iodine is an essential element required for the synthesis of these thyroid hormones. $A$ deficiency of iodine in our diet results in hypothyroidism and enlargement of the thyroid gland (goitre).
While both statements are scientifically true,the reason (the requirement of iodine) does not explain why the gland is composed of follicles and stromal tissue (which is a structural description). Therefore,the reason is not the correct explanation of the assertion.

(B) Thyroxine is a hormone secreted by the thyroid gland that plays a crucial role in the development of the central nervous system and the regulation of metabolism.
Deficiency or hyposecretion of thyroxine during fetal development or early childhood leads to a condition known as congenital hypothyroidism or cretinism.
Symptoms of cretinism include stunted physical growth,mental retardation,and deaf-mutism.
Therefore,the correct answer is hyposecretion of thyroxine.

(B) The incorrect statement is $B$.
$T_3$ (triiodothyronine) and $T_4$ (thyroxine) hormones are synthesized exclusively by the follicular cells of the thyroid gland.
Stromal tissues (connective tissues) do not synthesize these hormones.
Pars intermedia is indeed almost merged with pars distalis in humans.
Hypothyroidism during pregnancy leads to cretinism,characterized by stunted growth and mental retardation in the fetus.
Parathyroid hormone $(PTH)$ increases blood calcium levels by stimulating bone resorption,renal reabsorption of $Ca^{2+}$,and intestinal absorption of $Ca^{2+}$.

(A) The symptoms described,such as stunted growth (dwarfism),mental retardation,low intelligence quotient $(IQ)$,and abnormal skin,are characteristic of congenital hypothyroidism,also known as cretinism.
This condition occurs due to the deficiency of thyroid hormones ($T_3$ and $T_4$) during fetal development or early infancy.
Thyroid hormones are synthesized by the follicular cells of the thyroid gland.
Therefore,atrophy or dysfunction of the thyrofollicular cells leads to a lack of thyroid hormone production,resulting in these developmental abnormalities.
Thus,the correct option is $A$.

(A) The thyroid gland secretes two main types of hormones: $T_3$ (triiodothyronine) and $T_4$ (thyroxine),which are iodinated derivatives of the amino acid tyrosine,not steroid hormones.
Steroid hormones are typically secreted by the adrenal cortex,gonads,and placenta.
Therefore,the statement that the thyroid gland secretes only steroid hormones is incorrect.
$Hypothyroidism$ (low thyroid hormone levels) often leads to an enlargement of the thyroid gland,a condition known as goitre.
Thyroxine plays a role in maintaining water and electrolyte balance in the body.
$TCT$ (Thyrocalcitonin) is a protein hormone secreted by the thyroid gland that regulates blood calcium levels by lowering them.

(C) The thyroid gland secretes a protein hormone called Thyrocalcitonin $(TCT)$.
$TCT$ plays a crucial role in regulating blood calcium levels.
It acts as an antagonist to the Parathormone $(PTH)$ secreted by the parathyroid glands.
While $PTH$ increases blood calcium levels,$TCT$ lowers the blood calcium level by inhibiting bone resorption and promoting calcium deposition in the bones.

(A) Graves' disease is a form of hyperthyroidism caused by the overproduction of thyroid hormones.
It is clinically known as Exophthalmic goitre because its primary symptoms include the enlargement of the thyroid gland (goitre) and the protrusion of the eyeballs (exophthalmos).
Other symptoms include increased $BMR$,weight loss,and rapid heartbeat.
Since the symptoms mentioned in the Reason directly define why the disease is called Exophthalmic goitre,the Reason is the correct explanation of the Assertion.

(B) Statement $(A)$ is correct: Hypothyroidism during pregnancy leads to defective development and maturation of the growing baby,causing stunted growth (cretinism),mental retardation,and low intelligence quotient.
Statement $(B)$ is correct: Thyroxine is a lipid-soluble hormone (iodothyronine) that can cross the plasma membrane and bind to intracellular receptors. Therefore,it does not require a secondary messenger.
Statement $(C)$ is correct: Thyroxine plays a major role in the regulation of the Basal Metabolic Rate $(BMR)$ by stimulating the consumption of oxygen and the production of heat.
Statement $(D)$ is incorrect: Hypersecretion of thyroxine (hyperthyroidism) leads to conditions like Graves' disease,which is an autoimmune disorder,but it does not directly cause immunodeficiency.
Thus,statements $(A)$,$(B)$,and $(C)$ are correct. The total number of correct statements is $3$.

(A) Hyperthyroidism is a condition caused by the over-secretion of thyroid hormones ($T_3$ and $T_4$).
$1$. Increased $BMR$ (Basal Metabolic Rate): Thyroid hormones regulate metabolism; excess hormones lead to an increased $BMR$.
$2$. Protrusion of eyeballs: This is a characteristic symptom of Graves' disease,a form of hyperthyroidism (exophthalmic goitre).
$3$. Enlargement of the thyroid gland: Hyperactivity often leads to the swelling of the gland (goitre).
$4$. Increased body weight: This is incorrect. Hyperthyroidism leads to rapid weight loss due to the increased metabolic rate,not weight gain. Therefore,increased body weight is not a characteristic of hyperthyroidism.

(D) The symptoms described,such as low $BMR$ (Basal Metabolic Rate),thick tongue,prolonged neonatal jaundice,lethargy,and constipation,are characteristic of $Cretinism$.
$Cretinism$ is a condition caused by congenital hypothyroidism,which is the deficiency of thyroid hormones during fetal development or early infancy.
$Myxoedema$ is typically associated with hypothyroidism in adults.
$Cushing's$ disease is caused by excess cortisol.
$Addison's$ disease is caused by the deficiency of adrenal cortex hormones.

(B) The thyroid gland produces two main hormones: triiodothyronine $(T_3)$ and tetraiodothyronine ($T_4$,also known as thyroxine). Thyrocalcitonin (calcitonin) is also secreted by the thyroid gland to regulate calcium levels. Tyrosine is an amino acid that serves as a precursor for the synthesis of thyroid hormones,but it is not a hormone itself.

(A) . Addison's disease is caused by the hyposecretion of glucocorticoids and mineralocorticoids from the adrenal cortex. This leads to a decrease in the concentration of electrolytes (like $Na^+$) in the extracellular fluid,not an enhancement. Therefore,this option is incorrectly matched.
$B$. Graves' disease is a form of hyperthyroidism characterized by exophthalmos,which involves the deposition of fats and fluid in the eye sockets.
$C$. Myxoedema is caused by hypothyroidism in adults,leading to the thickening and puffiness of the skin.
$D$. Cretinism is caused by hypothyroidism during fetal development or early childhood,resulting in stunted physical growth and mental retardation.

(D) Graves' disease is an autoimmune disorder that results in hyperthyroidism,which is the overproduction of thyroid hormones.
Symptoms of Graves' disease include:
$1$. Enlargement of the thyroid gland (goiter).
$2$. Weight loss despite increased appetite.
$3$. Exophthalmos (protrusion of the eyeballs) due to the deposition of fats in the eye sockets.
$4$. Increased heart rate,anxiety,and heat intolerance.
Weight gain is a symptom of hypothyroidism (underactive thyroid),not hyperthyroidism. Therefore,weight gain is $NOT$ a characteristic of Graves' disease.