Thyroid Gland Hormones

What hormones are released by the thyroid gland?

Thyroid hormone (T3/T4) and calcitonin

What are the two forms of thyroid hormone?

T4 (thyroxine) - inactive form with 4 iodine atoms; T3 - active form with 3 iodine atoms

Which form of thyroid hormone is active?

T3 (3 iodine atoms)

Which form of thyroid hormone is inactive?

T4 (thyroxine, 4 iodine atoms) - must be converted to T3

What is thyroid hormone's target?

Most body cells

What are the three major effects of thyroid hormone?

1. Increases basal metabolic rate and heat production 2. Regulates tissue growth and development 3. Maintains blood pressure (by increasing adrenergic receptors)

What is an important component in synthesis of active thyroid hormone?

IODINE

What mechanism does thyroid hormone use?

Lipid-soluble, enters nucleus, direct gene activation

Describe the negative feedback mechanism for thyroid hormone

Low TH → hypothalamus releases TRH → anterior pituitary releases TSH → thyroid produces TH → High TH inhibits TSH (negative feedback)

What can override thyroid hormone negative feedback?

TRH from hypothalamus during pregnancy or cold exposure (especially in infants)

What is myxedema and what causes it?

Hypothyroidism in adults caused by low thyroid hormone; symptoms: water retention, low metabolic rate, chills, mental sluggishness, lethargy

What can cause a goiter?

Iodine deficiency - thyroid enlarges trying to make hormone without enough iodine

What is Graves' disease?

Most common hyperthyroidism; autoimmune disease where body makes abnormal antibodies that attack thyroid

What are symptoms of Graves' disease?

Elevated metabolic rate, sweating, rapid/irregular heartbeat, nervousness, weight loss despite eating, exophthalmos (bulging eyes)

What is exophthalmos?

Bulging eyes (symptom of Graves' disease/hyperthyroidism)

What is calcitonin's target organ?

Bones (specifically osteoclasts)

What triggers calcitonin release?

HIGH blood Ca2+ levels

What is calcitonin's action?

LOWERS blood Ca2+ by inhibiting osteoclast activity (stops bone breakdown); stimulates Ca2+ uptake into bone matrix

What does "calci-down-in" mean?

Memory trick: calcitonin brings calcium DOWN

What condition might warrant prescribing calcitonin to an older woman?

Osteoporosis (bone-sparing effect - prevents bone breakdown)

Does calcitonin have a known role at normal levels in humans?

No - only at higher-than-normal doses