Thyroid

Thyroid Macroanatomy

The thyroid gland Endocrine gland regulating metabolism through thyroid hormones. has two lobes. These are the right lobe and the left lobe. They are connected by a small structure called the isthmusA narrow portion of the uterine tube connecting to the uterus.. It is located anteriorThe front of the body or toward the front when standing in the anatomical position. to the trachea and inferiorBelow or toward the lower part of the body. to the larynxThe voice box; contains vocal cords and connects the pharynx to the trachea., with its edges curving slightly to the back. Many people forget that the thyroid gland generates two hormones: the thyroid hormone (TH): Includes T3 and T4, regulating metabolism. and calcitoninA hormone from the thyroid that lowers blood calcium levels by inhibiting osteoclasts..  Calcitonin, as the name suggests, plays a part in calcium regulation and is discussed in more detail in another minilecture.  This minilecture will focus on the thyroid hormone.

Thyroid Microanatomy

The thyroid histology is distinctive. The gland is made up of large circular structures called thyroid folliclesStructures in the ovaries that contain developing oocytes.. Inside each follicle is a clear substance called colloidA mixture where small particles are dispersed but not dissolved in a liquid., which stores thyroglobulin, a precursor to thyroid hormone. The follicles are surrounded by cuboidal epithelial cellsThe basic structural and functional units of life., known as follicular cellsCells surrounding the oocyte that support its development.. Where each follicular cell touches the colloid, thyroid hormone is being secreted.  You never EVER want to be without the thyroid hormone.  It is what prods all our cells to live, basically.  Unique to the thyroid gland is the storage of excess hormone in the colloid.  This storehouse of thyroid hormone could be mobilized when and if your follicular cells ever stop making TH.

Between the follicles are parafollicular cells or C cells. These C cells are responsible for producing calcitonin, a hormone involved in calcium regulation, rather than thyroid function.

Chemical Structure of TH

The thyroid hormone consists of two forms, T3 and T4, which can be measured in the blood. Thyroid hormone behaves as if it is nonpolar. In reality, it is polar. It binds to a protein called thyroid-binding globulin while in the bloodstream.  There are a few other transporters.  With such an important hormone, you want to be sure that it can circulate in blood.  With a few carriersMembrane proteins that transport substances across a cell membrane., TH is not dependent upon just one of them. 

The thyroid hormone is removed from circulation by the kidney.  There is a small range for the thyroid hormone in the blood.  The release of TH from colloid and the removal by the kidney are closely matched to maintain that equilibrium.

Effects of Thyroid Hormone

Thyroid hormone affects nearly all cells in the body. It targets cell nucleiClusters of neurons in the CNS responsible for processing information. and mitochondria to increase ATPThe energy currency of cells used for muscle contraction. production. This process raises metabolismThe sum of all chemical reactions in the body.. This is a very general statement: “raising metabolism.”  Ultimately, the thyroid hormone causes your cells to make ATP.  In order to do that, they need glucoseA simple sugar that is the main source of energy for cells. and oxygen.  This is what is meant by “metabolism” or your energyThe capacity to do work or cause change. usage.  The thyroid hormone increases your ATP production, raising your demand for glucose and oxygen.  So, your respiratory rate goes up and you breathe more deeply in an effort to bring in more oxygen.  Your hypothalamusA small but vital brain region controlling hormones, temperature, and autonomic functions. increases your feelings of hunger and you start to harvest energy from adipose deposits to fuel your mitochondria. 

Thyroid hormone has a permissive effectWhen one hormone enhances the effects of another.. It makes cardiac muscle more sensitive to epinephrineadrenaline): Fight-or-flight hormone from the adrenal medulla.. This happens by increasing the number of epinephrine receptorsProteins located on the surface or inside cells that bind specific molecules (e.g., neurotransmitter.  This accounts for the sign of increased heart rate with hyperthyroidism.

Thyroid Feedback Loop

The hypothalamus monitors blood levels of thyroid hormone and releases thyroid-releasing hormone(TRH): Stimulates TSH and PRL release. (TRH), which travels to the anterior pituitary glandEndocrine gland at the brain’s base controlling many hormones.. TRH, like all releasing and inhibiting hormones, only exists in the hypophyseal portal systemBlood vessel network connecting hypothalamus to anterior pituitary.. It can never be measured in your blood.  If you have a thyroid issue, we can never tell what your TRH is doing.  We can only assume. 

Cells called thyrotrophs in the APG are prompted by TRH to release the thyroid-stimulating hormone (TSH): Stimulates thyroid hormone production. (TSH).  TSH enters the bloodstream and circulates until it hits the thyroid organ.  There, it targets the colloid to release TH. It also prompts the follicular cells to manufacture more TH to replace the TH that is released.  This hormonal cascade exemplifies a negative feedbackA control mechanism that reverses a change in the body to maintain homeostasis. loop. Both the hypothalamus and anterior pituitary can inhibit further production of thyroid hormone when levels are sufficient. 

Thyroid Disorders

Thyroid disorders include hyperthyroidism and hypothyroidism.  These diseases can be caused by many things.  The most common cause of hyperthyroidism is Grave’s disease.  In this disease, auto-immune antibodies that have the same shape as TSH are made by some malfunctioning bone marrow.  These antibodies circulate and attach to the receptors on the follicular cells in the thyroid gland.  Even if TH levels are high, these antibodies will cause the thyroid gland to continue to release TH.  The TH in the blood is high. The hypothalamus thinks, “Well, I’m not needed here.” It shuts down TRH production. This, in turn, shuts down TSH production.  In this disease, TH remains high, but TSH is low.  What is TRH?  I don’t know because I can’t test for it.

An endemic goiter is an example of what happens with hypothyroidism.  This form of hypothyroidism is a product of a diet low in iodine.  The thyroid hormone is the only substance made from iodine in the human body.  If iodine is not obtained in the diet, you will not be able to make TH.  So, now, your hypothalamus senses the low TH and releases TSH.  TSH affects those thyrotrophs in the APG to release TSH.  But, in this disease, TSH connects with the follicular cells, but nothing happens.  The thyroid gland is like, “I can’t make TH.  No iodine.”  But, the APG is not hearing this and keeps sending TSH.  TSH can encourage those follicular cells to release TH. It can also cause growthAn increase in size and number of cells. of those follicles in the thyroid gland.  So, the thyroid gland adds more follicular cells, thinking it will make the TH the APG desires.  No.  No iodine.  However, the APG keeps sensing low TH levels. It prompts the thyroid gland to continue adding more follicular cells, even though it doesn’t help.  It’s the addition of these excess cells that produces a goiter.  Just a note that there are more diseases than these two, such as Hasimoto’s disease. 

Treating Thyroid Disorders

Treatment for hypothyroidism involves thyroid hormone replacement therapy.  I believe the drug is Synthroid, which is a synthetic version of TH.  I have heard some people say that Synthroid has lots of side-effects. They claim the natural version of the drug does not have these side-effects.  Just adding this drug does not cure your hypothyroidism in a pinch.  It can take months to years for the three organs to figure out how to work with each other.

Hyperthyroidism treatments are all dependent on one theoryA well-tested and widely accepted explanation.: remove the excess follicular cells and thyroid production will decrease.  Surgery can immediately remove cells.  The use of radioactive iodine can also kill follicular cells.  Yes, we intentionally give you radioactive iodine.  And you can’t hold babies, small children, or your pets for the 72 hours you are radioactive.  I have to have radioactive PET scans and I’m not supposed to pet my cats for 24 hours after.  Of course, when I come home from the PET scan, they both want my love.  OK, so radioactive substances are usually bad. However, radioactive iodine is effective. Thyroid cells are the only cells in the body that absorb iodine. I can have this radioactive iodine float around your body and only follicular cells will take it up.