38: Thyroid Gland Flashcards
What cell type secretes thyroid hormones
Follicular epithelium
Outermost part of thyroid follicle
Basal lamina
Follicular lumen contents
Contains colloid, which contains newly synthesized thyroid hormones attached to thyroglobulin, which stores iodine as tyrosine
How much hormone is stored in the thyroid + how much is secreted daily
Stored: enough for 2-3 months
Ssecrete: 60 ug daily
Parafollicular / C cell function
Secrete calcitonin
Name of T3 and T4
T3: triiodothyronine
T4: tetraiodothyronine
Which is the active form, T3 or T4?
T3
Which is secreted more, T3 or T4
T4 (10x more)
Half life of T3 and T4
T3: 1 day
T4: 6 days
How much T3 comes directly from thyroid vs peripheral conversion
Thyroid: 1–20%
Conversion: 80-90%
Enzyme that converts T4 to T3 in circulation
Deiodinase
Three things that reduce peripheral conversion to T3
- Fasting
- Medical stress
- Catabolic disease
Pendrin
A Cl/I pump involved in thyroid hormone synthesis, in the apical membrane of follicular cells
Mutation in Pendrin gene / Pendren syndrome
Defect in transport of iodine across apical membrane -> hypothyroidism with goiter + sensorineural hearing loss
Wolff-Chaikoff effect
High levels of I inhibit organification and synthesis of thyroid hormones
What can deficiency of deiodinase mimic?
Dietary deficiency in Iodine
What thyroid hormone is favored to be created with a restricted availability of iodine?
T3 is favored
Three main thyroid hormone binding proteins in the blood + their percentages
- TBG: thyroxine-binding globulin: 70%
- TRR: transthyretin: 10-15%
- Albumin: 15-20%
Where is TBG synthesized?
Liver
T3 Resin Uptake Test function
Measures circulating levels of TBG to measure levels of bound hormone
How T3 resin uptake test works
- Start with TBG bound to T4 from serum
- Add unbound and labeled T3, which will bind unbound TBG sites
- Anti-T3 Ab resin adsorbs T3, separating out T3 from T4 - can be measured at this point
Level of T4 and T3 in high and low TBG states
High TBG: high T4, low T3
Low TBG: low T4, high T3
Levels of TBG and T3 in hepatic failure**
Low TBG, high T3
Hepatic failure effect on thyroid hormone
It causes a transient increase in free T3 and T4, followed by an inhibition of synthesis of T3 and T4
TBG and T3 in pregnancy
High TBG, low T3
Conditions that cause high and low TBG
High TBG: pregnancy
Low TBG: hepatic failure
Pregnancy and thyroid hormones
Decrease in free T3 and T4 -> increased synthesis of T3 and T4 - >increased total levels of T3 and T4, but since also high TBG, levels of free H levels are normal -> pt is euthyroid
Thyroid hormones cause synthesis of what proteins in most tissues?
Na/K ATPase, transport proteins, lysosomal and proteolytic enzymes, structural proteins
Three proteins up regulated by thyroid hormones in cardiac muscle
Myosin, B1-adrenergics, Ca ATPase
Five causes of goiter
- Grave’s disease
- Secondary hyperthyroidism
- Lack of iodine
- Sporadic hypothyroid
- Hashimoto’s
two things TSH does to the thyroid
Regulates secretion of thyroid hormones + BUT also regulates growth of the gland
Secretion of Thyroid hormone vs GH
Thyroid: secreted in a steady state
GH: secretion in waves
Plasma cholesterol and triglycerides vs thyroid hormone
Inversely related
Neural changes that cause congenital hypothyroidism
Abnormal development of synapses + decreased dendritic branching and myelination -> neural changes irreversible unless replacement therapy starts soon after birth
