The pineal and thyroid glands · Biology Class 11–12 | StemPy Lab
Course 18.1·Lesson 3 of 3
The pineal and thyroid glands
The pineal gland makes melatonin, which sets the daily rhythm of the body. The thyroid gland makes thyroxine and triiodothyronine, which need iodine and control the basal metabolic rate. It also makes thyrocalcitonin. Too little iodine causes goitre, and too little thyroid hormone in a baby causes cretinism.
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The pineal gland
The pineal gland lies on the back (dorsal) side of the forebrain. It makes a hormone called melatonin.
Melatonin helps to regulate the 24 hour (diurnal) rhythm of the body. This includes the cycle of sleep and waking and the body temperature. It also influences metabolism, pigmentation, the menstrual cycle and the defence capability of the body. In general, the pineal gland releases more melatonin in the dark than in the light, and this is why bright light late at night can delay sleep.
The thyroid gland
The thyroid gland is made of two lobes on the two sides of the windpipe. A thin flap of tissue called the isthmus joins the lobes. The gland has many round sacs called follicles. Each follicle is lined by cuboidal cells and holds a store of hormone in the middle. The tissue between the follicles is the stroma.
Two drawings. On the left, the front of the neck: the windpipe in the middle, the thyroid with a right lobe and a left lobe joined by a narrow bridge called the isthmus, and four small oval parathyroid glands, two on the back of each lobe, shown with dashed outlines. On the right, a section of the thyroid with three round follicles. Each follicle has a wall of cuboidal cells around a cavity of colloid. The cells make thyroxine, T4, and triiodothyronine, T3, and iodine is shown entering the cells from the blood.
The follicle cells make two hormones.
Thyroxine, also called tetraiodothyronine or T4.
Triiodothyronine, also called T3.
The names count the iodine atoms: T4 has four and T3 has three. So iodine is essential for the normal rate of making these hormones. The thyroid also makes a protein hormone, thyrocalcitonin (TCT), which helps to regulate the level of calcium in the blood. A-Level and IB books call it calcitonin.
What thyroid hormones do.
They regulate the basal metabolic rate (BMR), the rate at which the body uses energy at rest.
They support the formation of red blood cells.
They control the metabolism of carbohydrates, proteins and fats.
They help to maintain the balance of water and electrolytes.
The control of the thyroid
A feedback loop in four boxes. The hypothalamus sends a releasing hormone to the pituitary. The pituitary sends TSH to the thyroid. The thyroid makes T3 and T4 and sends them to the body, where they raise the metabolic rate. Two red arrows with a minus sign run back from the thyroid hormones to the hypothalamus and the pituitary, to show that a high level of thyroid hormone switches off the release of the releasing hormone and of TSH.
The hypothalamus sends a releasing hormone to the pituitary. The pituitary sends TSH to the thyroid. The thyroid makes T3 and T4. When the level of T3 and T4 in the blood is high, they act back on the hypothalamus and the pituitary and switch down the release of TSH. This is negative feedback. It keeps the level of the hormones steady. TSH also makes the cells of the gland grow, which is important in the next section.
When the thyroid goes wrong
Too little thyroid hormone (hypothyroidism).
Iodine deficiency gives hypothyroidism and an enlarged thyroid. The enlarged gland is a goitre. With too little hormone, the feedback is weak, the pituitary sends more TSH, and the TSH makes the gland grow.
If a mother has hypothyroidism during pregnancy, the baby may be born with cretinism. The baby has stunted growth, mental retardation, a low intelligence quotient, abnormal skin and deaf-mutism.
In an adult woman, hypothyroidism can make the menstrual cycle irregular.
Too much thyroid hormone (hyperthyroidism).
A cancer or nodules of the thyroid can make too much hormone.
Graves' disease is a form of hyperthyroidism in which the body's own defence system attacks the gland. The thyroid is enlarged and the eyes bulge. This is called exophthalmic goitre. The metabolic rate rises, the person loses weight, sweats and has a fast heartbeat.
A graph made from the teaching model of the lesson. The horizontal axis is the iodine supply from 0 to 150 per cent of normal. The vertical axis is the level as per cent of normal. A blue curve for the thyroid hormones rises with the iodine supply and passes 100 per cent at an iodine supply of 100 per cent. A red curve for TSH falls as the iodine supply rises and passes 100 per cent at 100 per cent iodine. At low iodine the hormone is low and TSH is high, which makes the gland grow and gives a goitre.
You should find that a fall in the iodine supply lowers the thyroid hormone and raises TSH. In a real person the gland swells under the push of TSH. This is a goitre. Adding iodine, for example in iodised salt, prevents it.
The model is only a teaching device. Real levels are measured in the laboratory, and Graves' disease needs a different model, because there the gland ignores the feedback.
Try it in Python
The program follows the feedback loop step by step until the levels settle.
Practise with new numbers
Common mistakes
Saying that iodine is a hormone. Iodine is a part of T3 and T4. T4 has four atoms of iodine and T3 has three.
Saying that goitre comes from too much thyroid hormone. Simple goitre comes from too little iodine and too little hormone, with a high TSH.
Saying that melatonin comes from the pituitary. It comes from the pineal gland.
Mixing up TCT and the thyroid hormones. TCT is a protein hormone that helps to regulate the calcium of the blood. T3 and T4 contain iodine.
Saying that exophthalmic goitre has a low metabolic rate. It is hyperthyroidism, so the rate is high.
Remember these
The pineal gland makes melatonin, which helps to regulate the 24 hour rhythm: sleep, waking and body temperature.
The thyroid has two lobes joined by the isthmus. It makes T4 and T3, which need iodine, and the protein hormone TCT.
Thyroid hormones regulate the BMR. TSH controls the thyroid, and the thyroid hormones feed back to the hypothalamus and pituitary.
Hypothyroidism: goitre (iodine deficiency), cretinism (in the baby). Hyperthyroidism: Graves' disease with exophthalmic goitre.
Check yourself
Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.
What does melatonin help to regulate?
Name the two hormones of the thyroid that contain iodine. How many atoms of iodine does each hold?
Why does a lack of iodine in the food cause a swelling in the neck?
What is exophthalmic goitre, and what happens to the metabolic rate in it?
A baby is born with stunted growth and mental retardation because the mother had a thyroid disorder in pregnancy. What is the baby's disorder called?
References
Betts J. G. et al. (2022). Anatomy and Physiology 2e, OpenStax, chapter 17, The Endocrine System.
World Health Organization, UNICEF and ICCIDD (2007). Assessment of iodine deficiency disorders and monitoring their elimination, 3rd edition, Geneva.
Hall J. E. and Hall M. E. (2021). Guyton and Hall Textbook of Medical Physiology, 14th edition, Elsevier, the chapter on thyroid metabolic hormones.
NCERT (2023). Biology, Textbook for Class XI, chapter 19.
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