Thyroid hormone (thyroxine) is essential for metabolism, growth, and body-temperature regulation. The hierarchical feedback regulation of the hypothalamus-pituitary-thyroid axis is a basic model of endocrine regulation.
Basic Knowledge
Thyroid gland: A butterfly-shaped gland at the front of the neck. It secretes thyroid hormones (thyroxine T4 and triiodothyronine T3). These are amino-acid derivatives made from iodine.
Actions of thyroxine:
・Raises the basal metabolic rate of cells throughout the body (promotes mitochondrial oxidative phosphorylation and increases heat production).
・Necessary for growth and neural development (decreased thyroid function during infancy → cretinism: intellectual disability and short stature).
・Induces metamorphosis in amphibians (tadpole → frog).
Hypothalamus-pituitary-thyroid axis (HPT axis):
① Hypothalamus: Secretes TRH (thyrotropin-releasing hormone).
② Anterior pituitary: Receives TRH and secretes TSH (thyroid-stimulating hormone).
③ Thyroid gland: Receives TSH and secretes thyroxine.
④ Negative feedback: When thyroxine concentration rises, it acts on the hypothalamus and pituitary to inhibit TRH and TSH secretion.
Thyroid diseases:
・Hyperthyroidism (Graves' disease): Excess thyroxine → rapid heartbeat, weight loss, protruding eyeballs, and excessive sweating.
・Hypothyroidism (Hashimoto's disease): Insufficient thyroxine → fatigue, edema, low body temperature, and constipation.
・Goiter: Iodine deficiency → enlargement of the thyroid gland (the thyroid enlarges as compensation).
Answer: In the negative feedback of the HPT axis, when the blood concentration of thyroxine (T3/T4) rises, secretion of TRH by the hypothalamus and TSH by the anterior pituitary is inhibited, reducing thyroxine synthesis and secretion by the thyroid gland. Conversely, when thyroxine concentration falls, TRH and TSH increase and thyroxine secretion is promoted. In iodine-deficient areas, a shortage of iodine, the raw material for thyroid hormones, lowers thyroxine concentration. Negative feedback is therefore released, and TSH remains chronically high. Because TSH continues to stimulate the proliferation and enlargement of thyroid cells, a goiter forms.
- Thyroxine = secreted by the thyroid gland; promotes basal metabolism and growth
- HPT axis = hypothalamus (TRH) → pituitary (TSH) → thyroid gland (T3/T4)
- Negative feedback = increased thyroxine → decreased TRH and TSH
- Graves' disease = hyperthyroidism (rapid heartbeat, weight loss, protruding eyeballs)
- Hashimoto's disease = hypothyroidism (fatigue, edema, low body temperature)
- Frog metamorphosis = induced by thyroxine
Practice
- State, in order, the names of the hormones in the three stages of the HPT axis (hypothalamus → pituitary → thyroid gland).
- Explain why body weight decreases in Graves' disease based on the action of thyroxine.
- Explain why thyroxine is necessary for neural development during infancy, and describe cretinism.