Patients often present to ENT (Ear, Nose and Throat) doctors
with thyroid problems. We ENT docs
become involved mainly for surgery of the thyroid gland, which essentially involves
removing a part or the whole gland. We’ll
cover thyroid surgery (with drawings and some juicy surgery photos) in a future
post. Discussion of all things thyroid can
be lengthy, so to enhance your understanding and prevent undue boredom, I’ll break
this topic into several sections. We
should first talk about fundamentals. So
this post covers form and function, better known as anatomy and physiology
in medical parlance.
Where is the thyroid gland?
The thyroid gland is found in the lower, front part of the
neck. It’s a dumb-bell or butterfly-shaped
gland comprised of right and left lobes connected by a band of thyroid tissue
called the thyroid isthmus (think of Panama when you hear isthmus,
a thin strip of land connecting North and South America). The isthmus overlies
the trachea—the “windpipe”—and each lobe lies on either side of the trachea. Skin, fat, a thin platysma muscle and
then strap muscles cover the gland.
The strap muscles raise and lower the larynx (“voice box”) when
speaking or swallowing. The platysma
muscle allows you to tense the front of your neck, which for most folks
serves a noncritical function. The recurrent
laryngeal nerves run deep to each thyroid lobe. These nerves insert into each side of the
larynx and stimulate movement of the vocal cords, necessary for voice
production. There are also four parathyroid
glands lying on the back of each thyroid lobe, two on each side. The parathyroid glands regulate your body’s
calcium.
The thyroid gland itself is a soft structure that’s fed by
several small arteries and drained by several veins. The carotid artery (the major vessel that
sends blood to the brain) and jugular vein (the major vein the drains the head
and brain) are in close proximity to the gland.
Given its rich blood supply, the gland is very vascular. It is also surrounded by fat tissue containing
small lymph nodes (you can check out the description of lymph nodes and
the lymphatic system in Neck
Mass Evaluation and Workup).
Side-Bar: What
the heck is a goiter? You’ve
probably heard this term, which is a rather generalized term with no reference
to its cause. Goiter means an
enlarged thyroid gland, in the old days commonly caused by iodine
deficiency. Some folks mistaken the term
goiter with “tumor,” but in many cases it is not. Thyroid enlargement can result from a variety
of causes, such as a large nodule or several nodules, a benign mass or a cyst
(fluid filled space) within the gland.
However, “nodules” can be benign (noncancerous, non-worrisome) or
malignant (cancerous), the majority being benign. The gland itself can be large
without the presence of a nodule or mass.
We’ll cover more about thyroid abnormalities in future posts.
What does the thyroid do?
A “gland” is an organ that secretes something and the
thyroid gland secretes thyroid hormones.
There are two forms of thyroid hormone, abbreviated T4 and T3. We’ll not get into detail here only to say
that both serve similar functions (to satisfy the more curious out there, the
T4 molecule contains four iodine atoms and T3 has three). Their function is difficult to explain
succinctly in layman’s terms, but I’ll give it a shot. These hormones help regulate other body
systems to perform properly. If you’re
lacking thyroid hormone you can feel lethargic and depressed, gain weight, lose
hair, feel cold, amongst other things.
This is called hypothyroidism,
where hypo- is a fancy prefix in
medicine meaning “low” (leave it to doctors to create a longer word to describe
one with fewer letters). On the flip side,
hyperthyroidism is an overactive
thyroid, one that creates too much thyroid hormone. In this condition, you’d might lose weight,
experience tachycardia (elevated heart rate), feel hot and jittery and anxious;
much like an overdose of caffeine (if there’s such a thing as too much caffeine in this day and age).
How does it function?
Now let’s make things even more confusing and talk about
another hormone called thyroid
stimulating hormone (TSH for short).
TSH is secreted by the pituitary gland, a gland attached to the
brain. TSH circulates in the blood
stream, and as its name implies, it stimulates the thyroid gland to produce
more thyroid hormone. This process
causes the gland to absorb iodine circulating in the blood and causes thyroid
follicles (the fundamental “gland” unit in the thyroid) to start producing more
thyroid hormone (T3 and T4). The thyroid
gland needs a supply of iodine from your diet to create T3 and T4. People who say they have an iodine are not
truly allergic to the iodine molecule itself, but to a variety of other
compounds of which iodine is one part.
As more T3 and T4 circulate in the bloodstream, the
pituitary gland senses this increase and through a process called a negative feedback loop, the pituitary then
slows down its secretion of TSH. The
decrease in circulating TSH then signals the thyroid gland to slow down its
production of T3 and T4. Hence, there’s
an inverse relationship between TSH
and the T3 and T4 levels; when TSH is high, the body is hypothyroid or low in T3 and T4. When TSH is low, there is a state of hyperthyroidism, where there’s too much
T3 and T4. If TSH is in the normal
ranges, then you are euthyroid—meaning your thyroid T3 and T4 are at
normal levels. TSH happens to be a very
sensitive test to diagnose whether one is hypo-, hyper- or euthyroid, more so
than measuring the actual T3 and T4 levels, which often can be normal in lesser
degrees of hypo- or hyperthyroidism.
This is the reason your doctor orders blood work to check your TSH level
as part of the workup to assess the function of your thyroid gland.
Many patients mistakenly assume that if their thyroid labs
are normal (ie, TSH, T3 and T4) then there is nothing in their thyroid
gland. However, oftentimes nodules or
tumors can arise in the gland resulting in absolutely no change in thyroid
function.
Are there other methods to further assess the thyroid?
Yes. The key is to
dive deeper, so to speak, to determine the cause of an enlarged thyroid. One of the most useful tools is a thyroid
ultrasound.
Ultrasound uses sound
waves with frequencies that are higher than the human ear can detect. Thus, the fancy prefix ultra-, meaning “beyond” or “excess.” Ultrasound is a relatively safe mode of
imaging since it uses no radiation. The
ultrasound device transmits these sound waves into the tissues and measures the
response as they are reflected back. A
fancy computer runs calculations (that are far beyond my expertise to explain)
to create meaningful images.
Below is an example of a normal thyroid ultrasound
image.
This image is an axial
cross-section—imagine slicing the neck horizontally and then looking up
into it.
We’ll get into abnormal U/S findings when we talk about not-so-normal
thyroid findings in future posts, but I’d like to delve into one further item
here. In the case of nodules, patients
often ask, “Hey doc, why not remove the nodule only? Leave the rest my gland alone!” As mentioned before, the thyroid is very
vascular, much like the spleen. Nodules
are typically deep within the gland. Due
to its soft and vascular nature, it’s nearly impossible to localize and remove
a nodule or mass within the thyroid gland.
Opening the gland by cutting through its capsule and digging within its
substance is a bloody mess. The bleeding
also could not be well-controlled, necessitating removal of the gland anyway. So to remove the nodule, one needs to remove
entire thyroid lobe containing it.
Anyway, congratulations!
You just learned something about thyroid anatomy and physiology. Tell that to your family and friends at the
next social event, or use the pick-up line in the opening illustration to
really impress that special someone; it’s all in the delivery. However, you may need a drink or two beforehand
as an excuse should you fail.
Randall S. Fong, M.D.
For more topics on medicine, health and the weirdness of
life in general, check out the rest of the blog site at randallfong.blogspot.com
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