☰ Year plan · Biology

Thyroid, adrenal, pancreas and the other glands

Week 5 · Chapter 2 (B): thyroid and parathyroid glands, the adrenal glands (cortex and medulla), the pancreas (insulin, glucagon, diabetes), the sex glands and gut hormones, and a table of the glands (Darslab's own) · 3 lessons
1

Thyroid and parathyroid: iodine, metabolism and calcium

Biology Sec 3 Term 1, Chapter 2: pp. 29–35
GoalDescribe what the thyroid and parathyroid glands make, and how they keep metabolism and blood calcium in balance.
New words
thyroxine · the thyroid hormone that controls growth, development and the rate of metabolism; the thyroid needs iodine to make itiodine · a mineral in food, water and air that the thyroid needs in order to make thyroxinecalcitonin · a thyroid hormone that lowers the calcium level of the bloodparathormone · the parathyroid hormone that raises the calcium level of the blood when it is below normal
Explanation

The thyroid gland lies in the neck in front of the windpipe. It has two lobes joined by a narrow bridge, the isthmus. It makes thyroxine, which the gland can only do with iodine. Thyroxine affects physical and mental growth and development, sets the basal metabolic rate, increases the absorption of simple sugars from the intestine, and helps keep skin and hair healthy. The thyroid also makes calcitonin, which lowers the calcium level of the blood and stops calcium being withdrawn from the bones. Too little thyroxine is hypothyroidism. Its common cause is a shortage of iodine in food, water and air, which makes the gland swell: simple goitre. Iodine added to salt and food prevents it. A severe lack of thyroxine causes cretinism, when it happens in childhood and slows growth and development, or myxoedema, when it happens in an adult and slows the metabolism. Too much thyroxine is hyperthyroidism, which speeds the metabolism up: in exophthalmic goitre the gland is enlarged and the eyes bulge. This is biology, not a way to check a person: a doctor decides whether anyone has a thyroid problem. The four small parathyroid glands, two on each side of the thyroid, make parathormone when the blood calcium is below normal. Parathormone and calcitonin work together to keep calcium steady. Too much parathormone lets too much calcium leave the bones and raises the blood calcium, and too little lets it fall. Either way the balance that bones, muscles and nerves depend on is upset, and a doctor decides what is wrong.

HormoneMade byMain jobWhen out of balance
Thyroxinethyroid (needs iodine)growth, development, metabolic ratetoo little: simple goitre, cretinism, myxoedema; too much: exophthalmic goitre
Calcitoninthyroidlowers blood calcium—
Parathormoneparathyroidsraises blood calciumtoo much or too little: calcium balance upset
Worked examples
Blood calciumGland that respondsHormoneEffect
too highthyroidcalcitonincalcium falls and stays in the bones
too lowparathyroidsparathormonecalcium rises, released from bone
Keeping calcium steady, like a thermostat with a heater and a cooler. Two hormones push in opposite directions.
Why iodine matters. Think of a village far from the sea whose food and water hold very little iodine. The thyroid cannot make enough thyroxine, so many people may develop simple goitre. Iodine added to salt and food can prevent this.
ConditionCause
Simple goitreshortage of iodine, so too little thyroxine
Cretinismsevere lack of thyroxine in childhood
Myxoedemasevere lack of thyroxine in an adult
Exophthalmic goitretoo much thyroxine
Match each condition to its cause.
Class activity

Read a label, with a grown-up. Look at a packet of salt at home and find out whether it says iodised. Write why a country might add iodine to salt, and name the gland and the hormone that depend on it. Then draw a thermostat on a wall, label one arrow heater and one arrow cooler, and write which hormone is the heater and which is the cooler for blood calcium.

Practice
1
Which mineral does the thyroid need to make thyroxine?
2
Which hormone lowers the calcium level of the blood, and which gland makes it?
3
A long shortage of iodine in the diet makes the thyroid swell. Name this condition.
4
Why should a person not decide alone that the thyroid is the cause of a problem?