Video Transcript
Which of the following conditions
is characterized by an increase in thyroxine secretion that leads to the enlargement
of the thyroid, enlargement of the neck, and bulging of the eyeballs? (A) Hyperglycemia; (B) exophthalmic
goiter, Graves’ disease; (C) myxedema; or (D) congenital hypothyroidism.
This question is asking about a
condition caused by an excess of thyroxine. Let’s therefore review the key
facts about the thyroid gland, its secretions, and the conditions relating to an
imbalance of hormones.
The thyroid gland is a
butterfly-shaped endocrine gland found at the base of the neck. It secretes two main hormones,
thyroxine and triiodothyronine, which play an important role in growth, development,
and metabolic rate, as well as many other functions. To carry out their functions
properly, the level of these hormones needs to be controlled carefully. This is done by a negative feedback
mechanism, involving the hypothalamus and the pituitary gland.
Low blood thyroid hormone levels
are detected by the hypothalamus, which then releases a hormone called
thyrotropin-releasing hormone, or TRH for short. Thyrotropin-releasing hormone
travels to the pituitary gland and triggers the release of thyroid-stimulating
hormone, or TSH. The thyroid-stimulating hormone
then travels in the blood to the thyroid gland, where it binds to receptors on the
cells, stimulating the release of more thyroid hormones, bringing the level up to
the norm.
High blood thyroid hormone levels
are again detected by the hypothalamus, which releases less thyrotropin-releasing
hormone. So the pituitary gland releases
less thyroid-stimulating hormone, and so the thyroid gland slows secretion to bring
the level back down.
When the thyroid gland, or the
negative feedback mechanism, isn’t functioning properly, a person develops
characteristic symptoms and suffers adverse consequences. This question is asking about a
condition characterized by an increase in thyroxine secretion. So let’s have a look at each of the
choices given and narrow them down.
Starting with hyperglycemia, this
can be eliminated straight away as it is not even associated with the thyroid. It is a condition in which a
person’s blood sugar is too high, hyper- meaning high and glycemia referring to
glucose in the blood. It is most often associated with
diabetes.
We have now seen hyper- means high,
while hypo- means low. So hypothyroidism refers to the
underproduction of thyroid hormones. Congenital describes a condition
existing at or before birth. In infants and children, congenital
hypothyroidism is usually brought about by a lack of iodine in a mother’s diet early
on in pregnancy. One of the main constituents of the
thyroid hormones is iodine. So if the mother’s diet is low in
iodine, this will lead to her having hypothyroidism, resulting in irreversible
damage to the fetal brain. Obviously, a lack of iodine in the
mother’s diet means the developing child is also short of iodine and therefore also
ends up with hypothyroidism, impeding physical growth and development.
Myxedema refers to a severe form of
hypothyroidism in adults. This condition results in many
symptoms, including decreased heart rate, high blood pressure, seizures, and, if
left untreated, coma.
If we look back at the question, it
is asking about a condition where the thyroid hormones are being overproduced. So we can rule out options (C) and
(D) as these are referring to conditions where they are being underproduced.
This leaves us with exophthalmic
goiter, also known as Graves’ disease. This is an autoimmune disease which
causes hyperthyroidism, in other words, high levels of thyroid hormones. With this condition, the body
mistakenly attacks the thyroid gland, causing it to overproduce its hormones, as
well as attacking the tissue behind the eyes, causing them to swell. As a result, the eyes bulge and the
neck becomes enlarged.
We now therefore know that the
correct answer to our question is option (B). The condition that is characterized
by an increase in thyroxine secretion leading to the enlargement of the thyroid,
enlargement of the neck, and bulging of the eyeballs is exophthalmic goiter, Graves’
disease.
