7. Thyroid Hormone Physiology Flashcards
thyroid gland anatomy
bilobed, connected by isthmus
gland is composed of follicles
thyroid gland histology
each follicle consists of:
- monolayer of epithelial cells
enclosed large core of viscous homogenous colloid
what does colloid do?
acts as reservoir for thyroid hormone
thyroid gland hormones
triiodothyronine (T3)
tetraiodiothyronine (T4, thyroxine)
calcitonin
calcitonin
concerned with calcium homeostasis
secreted independently of other thyroid hormones
T3/T4 synthesis
MIT and DIT are rapidly degenerated by halogenates - frees iodine
iodine is combined with thyroglobulin
2 molecules of DIT make T4
~95% leaving thyroid gland is T4 - converted to T3 in tissues
differences between T3 and T4
T4 has one more iodine thyroid gland releases mostly (95%) T4 T3 is 40x more biologically active 90% inside cell - T3 T3 half life - 1 day T4 half life - 6-8 days
control of thyroid hormone secretion
hypothalamus releases TRH
TRH stimulates anterior pituitary to release TSH
TSH stimulated secretion of T3 and T4 by thyroid gland
thyroid hormones in blood
insoluble in water
bound to plasma proteins
99% = protein bound
T4 binding
75% to thyronine-binding globulin
15-20% to thyroxine prealbumin
5-10% to albumin
biological effects of thyroid hormones
increase basal metabolic rate increased carb metabolism increased lipid synthesis, mobilisation and degradation increased protein synthesis essential for normal CNS development
thyroid hormones mechanisms
increase number and size of mitochondria
increase activity to metabolically active enzymes
thyroid hormone does NOT increase O2 consumption in
brain uterus testes spleen thyroid gland ant. pituitary gland
drugs affecting thyroid function
carbimazole
propylthiouracil
potassium percolate
radioiodine
carbimazole
converted to methimazole in body
inhibits synthesis of thyroid hormones
prevents incorporation of iodine into thyroglobulin
