The thyroid gland is a part of the endocrine system. In fact, it's the largest endocrine gland in the body.
The thyroid is found at the base of the neck, underneath a cartilage shield.
It controls how quickly the body burns energy, how quickly it makes proteins and how much attention the body should pay to other hormones circulating in the blood.
The thyroid gland controls how much cholesterol is in the blood, how much insulin gets released to deal with blood glucose, where fat goes in the body and how you're going to react to hormones like norepinephrine.
To do all of this, the thyroid gland produces thyroid hormones.
The vast majority of thyroid hormone produced is either thyroxine, commonly known as T4, and triiodothyronine, commonly known as T3.
These two hormones regulate the metabolic rate and affect the growth or function of many other bodily systems.
In order to make these two thyroid hormones, the body needs iodine. Iodine deficiency itself still leads to developmental problems for children in many parts of the world, and in adults can lead to hypothyroidism.
In older thyroid function tests, T3 and T4 alone would often be the only things tested for.
However, the thyroid function tests have advanced quite a bit since then, and often do a good job at mapping total thyroid function.
Calcitonin is the last hormone the thyroid produces, and it plays a vital part in the movement of calcium into and out of the bones. Calcitonin imbalances are extremely rare, indicating a serious form of thyroid cancer when it happens, but it's not usually tested for.
The thyroid is controlled by the hypothalamus and the pituitary gland, both located in the brain.
Production of T3 and T4 is specifically regulated by thyroid stimulating hormone, commonly shortened to TSH.
TSH is released by the back portion of the pituitary gland in response to thyrotropin regulating hormone (TRH) released by the hypothalamus. When the pituitary releases TSH, it travels through the bloodstream down to the thyroid gland and tells it to start producing T3 and T4.
TSH is usually regulated by the level of T4 in the blood. Under normal circumstances, TSH production is suppressed when T4 levels are high. However, if the brain decides that it wants to step up the metabolism, for instance it's really cold out and you need to burn internal fuel for heat, the hypothalamus will override the T4 levels by putting out increased TRH, telling the pituitary gland to get going with the TSH again. Mind you, that's how everything works when normal.
The symptoms of thyroid malfunction are reasonably distinctive if you know what to look for. However, if you don't, they can often get confused with other medical disorders, up to and including clinical psychiatric depression.
Hyperthyroidism:
The major symptoms of hyperthyroidism, or too high levels of thyroid hormones, are weight loss, increased appetite, anxiety, intolerance for heat, fatigue, hair loss, weakness, hyperactivity, irritability, apathy, depression, excessive urination, excessive thirst and drinking, and excessive sweating.
Hypothyroidism:
Hypothyroidism, on the other hand, often presents with poor muscle tone, fatigue, excessive sensitivity to cold, depression, constipation, muscle cramps, joint pain, arthritis, thin, brittle fingernails and hair, paleness, dry, itchy skin, weight gain, water retention, and the swelling of the thyroid gland.
Hypothyroidism & Hyperthyroidism:
The symptoms of hyperthyroidism and hypothyroidism, may also indicate thyroid cancer, as a cancerous tumor will generally either stimulate the thyroid gland or suppress it.
If the doctor believes your thyroid looks enlarged, you may be refferred for an ultrasound to confirm that opinion. The ultrasound will normally show one of three things:
1.. Thyroid appears normal
2.. Thyroid appears enlarged (thyroiditis)
3.. Thyroid has areas of enlargement on it (patchy thyroiditis).
The symptoms of patchy thyroiditis are the same as thyroiditis. Patchy thyroiditis can have the same effects as full blown thyroiditis. Most likely cause is considered to be an autoimmune response, with the body attacking the thyroid. This can happen, for example, if you have a cyst on a tooth. The infection is close to the thyroid, so the body attacks the thyroid as well. Controlling the infection may lead to the patchy thyroidits resolving.
When thyroid problems are suspected, doctors will usually perform a variety of thyroid function tests in order to determine exactly what the problem is and where it's coming from.
The first test these days is usually for TSH levels. High levels of TSH generally point to an inadequate production of hormones by the thyroid gland, while abnormally low TSH levels point to excessive hormone production by the thyroid.
If the TSH is abnormal, the next test performed will usually be for overall T3 and T4 levels in order to confirm the problem.
The biggest problems that a thyroid function test panel looks for are hypothyroidism, hyperthyroidism, cancer, or a problem with the pitutitary gland or the hypothalamus.
Usually these tests are only done when symptoms of thyroid problems come up, but some medications have been linked to thyroid disease, including amiodarone, lithium salts, and some varieties of interferon. Therefore, if you're on one of these medications, don't be surprised when your doctor wants thyroid function test panels done every so often just to make sure everything's still working correctly.
When either hyperthyroidism or hypothyroidism is suspected, your doctor will usually start the thyroid function test panel, commonly referred to as TFTs.
A typical TFT panel includes a test for thyroid stimulating hormone from the pituitary along with tests for the total levels of T3 and T4 in the blood, depending on the policies of the local laboratory. Further thyroid testing is done once the doctor has a good idea of what direction to go as given by the initial tests.
Thyroid function tests are also commonly performed to check on conditions known to be associated with thyroid dysfunction, such as atrial fibrillation, as well as to check on the performance of thyroid medication.
The normal range for TSH blood levels is between 0.3 to 3.0 uIU/mL. That's micro International Units per milliliter.
International Units are a measurement used for certain biologically active agents. Each was individually agreed upon by the medical board for multiple countries, and the exact amount varies per substance. The older measurement of normality was between 0.4 uIU/mL and 5.0 uIU/mL, but this has recently changed by medical consensus. Unfortunately, many laboratories haven't gotten the news yet, so they may still be going off the old measurements.
Basically, if you have TSH lower than 0.3 uIU/mL then you stand a chance of having hypothyroidism, and if it's above 3.0 uIU/mL, you could have hyperthyroidism.
However, at the point that an abnormality of any kind is noticed in TSH, the next step is to check the T3 and T4 levels.
I could not find any reputable guidelines for T3 and T4 hormone levels anywhere. Apparently, each and every laboratory has its own guidelines, and they're all subject to your doctor's interpretations.
However, there were a few tendencies I found to help you and your doctor figure out what the exact problem is and where in the body it's coming from.
TSH levels, T3 levels and T4 levels should all be measured to find out whether a problem is primarily cause by the pituitary/hypothalamus complex or primarily by the thyroid gland itslef.
If both TSH and thyroid hormone levels are high or low together, then the problem is probably in the brain.
If one is up and the other is down, then the problem is most likely in the pitutitary itself.
If the disease originates in the hypothalamus/pituitary gland, the TSH level is high and the thyroid hormone level is high then the diagnosis is probably either a benign tumor of the pitutitary or thyroid hormone resistance.
If the disease originates in hypothalamus/pituitary gland, the TSH level is low and thyroid hormone levels are low, then the diagnosis is probably is hypopituitarism.
If the disease originates in the thyroid gland itself, the TSH level is low and the thyroid hormone high, then the diagnosis is probably hyperthyroidism or Grave's disease.
If the disease originates in the thyroid gland, the TSH level is high and the thyroid hormone levels are low, then the diagnosis is probably hypothyroidism or congenital hypothyroidism caused by iodine deficiency.
Both TSH and T3 and T4 should be measured to find where a thyroid dysfunction is primarily caused by pituitary or primarily by thyroid. If both are up or both down, probably in the pituitary. If one up, one down, problem is probably in the thyroid.
There's a plethora of other tests for thyroid function, including direct measurements of free hormones, tests for carrier proteins such as thyroid binding globulin, transthyretin and albumin, and tests to measure the protein binding of thyroid hormones.
All of these tests are done if the symptoms of either hyperthyroidism or hypothyroidism are present, but the initial tests don't show any problems between the pitutary/hypothalamus and the thyroid gland itself. These tests are mostly to try and find out if thyroid hormones, which should be present in sufficient quantity, are instead getting chemically stuck in other places in the body.
However, keep in mind that there are a few thyroid malfunctions that don't show up on tests for some reason or another. Many testing parameters are so tight that they give "false negatives", which is to say that you do have a problem but the test says you don't.
If you have problematic symptoms, you need to communicate with your doctor in order to get to the base of the problem. It might be with your thyroid, it might not, but without open, clear and respectful communication, you can't work in partnership to find out what the actual disorder is. Once you find it, you can start to manage it.
Main Research and write by Loni Ice, Editing by Donald Urquhart