Module 2 lesson 4 - 5

Lesson Summary

This lesson explored the following focusing question:

• How do the thyroid and parathyroid glands contribute to homeostasis?

The thyroid gland, a butterfly-shaped gland located just below the larynx, has embedded in it the four pea-sized parathyroid glands. These glands produce two antagonistic hormones - calcitonin and PTH - that work together to maintain blood calcium homeostasis. Through mechanisms such as increasing absorption of calcium from food or urine and by stimulating decomposition of bone tissue, PTH increases blood calcium levels. Calcitonin decreases blood calcium levels by stimulating uptake of calcium by the bones and inhibiting decomposition of bone tissue. Both are regulated by negative feedback mechanisms.

When stimulated by TSH from the anterior pituitary, the thyroid gland produces thyroxine that increases metabolism (cellular respiration) in every cell of the body and especially in cardiac cells, muscle cells, and cells of the kidneys and liver. It stimulates the liver to convert glycogen to glucose and increases body temperature. Thyroxine also stimulates proper development and function of the nervous system. Iodine is required to synthesize thyroxine. Lack of thyroxine is called hypothyroidism and presents differently in babies/children and adults. Over-production is called hyperthyroidism. Goitre and cretinism are disorders caused by imbalances in thyroxine. Thyroxine levels are regulated by negative feedback of the tropic hormone TSH produced by the anterior pituitary. Surgical removal of parts of the thyroid, death of the most active follicle cells initiated by radioactive iodine, and consumption of thyroid pills are medical technologies that can treat thyroxine imbalances.