Introduction

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.

Page 1

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?

Secrete aldosterone
Secrete Corticotropin Releasing Hormone
Secrete Thyrotropin Releasing Hormone

Secrete Thyroid hormones

Page 2

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 3

The 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.

Page 4

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?

Stop making thyroid hormones
make more TRH and TSH
make less TRH and TSH

Is this negative or positive feedback?

Positive, because it stimulated the thyroid

Negative, because the stimulus from the hypothalamus and pituitary decreased when the variable reached the set point

Page 5

What are T3 and T4, anyway? Let's take the names apart.

Tri = three
iodi = iodine
thyronine = an amino acid

So Triiodothyronine is the amino acid thyronine with 3 iodine atoms stuck onto it.

What's Tetraiodothyronine?

Four T3 molecules fastened together

Four iodine atoms attached to a T3

Four iodine atoms fastened to a thyronine

Page 6

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:

Now you know this, you should be able to figure out what high and low thyroid hormone levels will do to a person.

Page 7

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?

Hyperthyroidism
Hypothyroidism

The doctor orders blood tests, and they show low T3 and T4. But her TSH is elevated. Why would this happen?

If TSH is elevated, there must be too much TRH
her anterior pituitary is overactive
her hypothalamus and anterior pituitary are responding normally to the low T3 and T4

What organ is most likely working wrong, to cause her low T3 and T4 levels?

her thyroid
her anterior pituitary
her hypothalamus

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?

TRH
TSH
T3
T4

Page 8

Mrs. 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?

her thyroid is more severely damaged
her anterior pituitary isn't working properly
her hypothalamus isn't working properly

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
If she responds, it will confirm that the problem was with her hypothalamus
If she responds, it will confirm that her problem is in the anterior pituitary

Page 9

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 problem is in her thyroid - primary hypothyroidism
The problem is with her hypothalamus - tertiary or central hypothyroidism
The problem is in her anterior pituitary - secondary hypothyroidism.

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?

Mrs. A can make TRH, and Mrs. B can't
Mrs. A can make TSH, and Mrs. B can't

"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
Measuring TSH lets the doctor know whether her anterior pituitary is still working
Her anterior pituitary is working, and it will adjust TSH according to whether T3 and T4 levels are too high or too low.

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.

Happy studying!