Control of Thyroxine Levels - Negative Feedback Simplified Revision Notes for Leaving Cert Biology

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Control of Thyroxine Levels - Negative Feedback

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The thyroid gland;

Is found in the neck, in front of the larynx.
Produces thyroxine. Thyroxine controls metabolism. The pituitary gland
Often called the 'master gland' because its hormones cause other glands to release their hormones.
Found at the base of the brain, just below the hypothalamus, and is connected to the hypothalamus by a portal system.

Hormones Released by the Pituitary Gland:

Hormone	Function
Thyroid Stimulating Hormone (TSH)	Stimulates the thyroid gland to release thyroxine.
Follicle Stimulating Hormone (FSH)	Controls the formation of eggs in ovaries and sperm in testes.
Luteinising Hormone (LH)	Controls ovulation in females and testosterone production in males.
Growth Hormone (GH)	Causes cells to increase in size.
Anti-diuretic Hormone (ADH)	Regulates water balance (osmoregulation) in the kidneys.
Prolactin	Causes milk formation.
Oxytocin	Causes the uterus to contract during labour.

Control of Thyroxine Levels

The concentration of thyroxine, like all hormones, is regulated by a mechanism called negative feedback.

The pituitary gland produces TSH (Thyroid Stimulating Hormone), which prompts the thyroid gland to produce thyroxine.
When thyroxine levels in the bloodstream are normal, no TSH is produced, so no more thyroxine is made. This is the negative feedback mechanism at work.
If thyroxine levels drop below normal, the pituitary gland produces TSH, which then causes more thyroxine to be produced.
Once thyroxine levels return to normal, TSH production is inhibited, stopping further production of thyroxine.

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