This tutorial will introduce you to the thyroid, what it does and how it is regulated.
You will navigate through this tutorial using the buttons at the top of the screen.
The tutorial will ask you questions. Click on your chosen answer to see feedback; click the answer again to make the feedback disappear. When you're finished with one page, click the navigation button for the next page to move ahead.
Have fun! Click on button '1' to see the first page of the tutorial.
Thyroid hormones control your metabolic rate -- how fast you burn the food you eat and make ATP. That means that thyroid hormones are necessary for you to have energy and maintain your body temperature. You need to keep a certain level of thyroid hormones in your body at all times or your cells won't be able to work properly.
The hypothalamus is the part of your brain most closely involved with homeostasis, and it contains many different groups of cells that are responsible for measuring aspects of your body. One group of cells measures your thyroid hormones and compares their level to the set point.
If the hypothalamus detects low thyroid hormone levels, what will it do?
Releasing Hormones are secreted from the hypothalamus into a tiny network of blood vessels that run to the pituitary. These vessels are called the hypothalamohypophyseal portal system, or the hypophyseal portal system for short.
The hypothalamus contains many groups of cells that have specific functions. One group is in charge of measuring thyroid hormones, and secreting Thyrotropin Releasing Hormone if the thyroid hormones levels are too low. It will also secrete TRH when the body is cold, and during sleep.
Page 3The anterior pituitary contains many different groups of cells in charge of creating and storing hormones that can control other organs.
Releasing hormones from the hypothalamus tell the anterior pituitary which of these stored hormones to release.
In this case, the Thyrotropin Releasing Hormone will tell the anterior pituitary cells in charge of the thyroid to release Thyrotropin, also called Thyroid Stimulating Hormone, or TSH. The TSH will be released into the bloodstream and sent throughout the body - including to the thyroid gland.
Thyroid Stimulating Hormone will stimulate the thyroid gland to make two hormones:
Triiodothyronine (T3) and Tetraiodothyronine (T4 or thyroxine). The hypothalamus and pituitary cells in charge of the thyroid can measure the T3 and T4 levels and tell when they reach the set point. What will the hypothalamus and anterior pituitary do then?
Is this negative or positive feedback?
What are T3 and T4, anyway? Let's take the names apart.
Tri = three
So Triiodothyronine is the amino acid thyronine with 3 iodine atoms stuck onto it.
Although T4 was discovered first and named thyroxine, T3 is actually the active form of the thyroid hormone. Here are some of its effects on the body:
METABOLISM SPEEDS UP:Now you know this, you should be able to figure out what high and low thyroid hormone levels will do to a person.
1. More aerobic metabolism creates more ATP: more energy
Mrs. A has come to the doctor complaining of constipation. She has gained weight in the past month, though she says she hardly eats anything. She is wearing a winter coat even though it is August.
What thyroid imbalance does she most likely have?
The doctor orders blood tests, and they show low T3 and T4. But her TSH is elevated. Why would this happen?
What organ is most likely working wrong, to cause her low T3 and T4 levels?
The doctor has decided that Mrs. A has primary hypothyroidism, which means the problem is in the thyroid gland itself. The doctor wants to give her replacement thyroid hormones. Which hormone(s) do you think will be given?
Page 8Mrs. B is seeing the doctor, and she has almost identical symptoms. But her blood values are different: she has low T3 and T4, AND low TSH. What does this tell you?
The doctor prescribed T4 for Mrs. B, and her symptoms improved. But the doctor was concerned about what might be causing them, and sent Mrs. B to an endocrinologist.
The endocrinologist ordered an MRI scan of Mrs. B's brain, and something called a TRH test. "In this test, we inject you with TRH and see if you respond by raising your TSH and T3 and T4 levels."
What will that test tell them?
If she responds, it will correct her problem for good
Mrs. B's brain scan didn't show any abnormalities, and she did not respond to the TRH test. Her TSH and T3 and T4 levels remained low.
What does this mean?
The endocrinologist told Mrs. B that she should continue taking the T4 her doctor prescribed. "It's the simplest and easiest solution. But you have to have routine T4 tests to tell whether the levels are correct."
When Mrs. B went for a blood draw for her routine test, she met Mrs. A in the lab's waiting room and they discovered they had the same doctor and problem - but they were getting different tests to monitor it! Mrs. A was getting a TSH test, and Mrs. B was getting a T4 test. They wonder why different tests are used. Can you explain it?
"I can see why my test wouldn't work for you," Mrs. A says to Mrs. B, "But not why your test wouldn't be better for me. If they're giving us both T4, aren't the T4 levels the most important thing to measure?"
Why might her doctor be measuring TSH instead of T4?
T4 isn't the active form of the hormone, so its levels don't matter
You've finished this thyroid tutorial. As you study further, try to figure out what would happen to a person with too much thyroid hormone and how that could be caused. Also try and figure out what would happen to the hormone levels in a person whose diet contained no iodine at all.
Copyright Patricia S. Bowne firstname.lastname@example.org
Material for this activity is taken from:
Seeley, R.R. , Stephens, T.D., and P. Tate, 2003.Anatomy and Physiology, 6th Edition. McGraw-Hill Higher Education.