Phys 71:thyroid

Front 1

Describe the synthesis of thyroid hormone

Back 1

In the follicular cell, thyroglobulin (TG) is synthesized from tyrosine and extruded into the colloid. Meanwhile, I- is actively cotransported with Na into the follicular cell. Peroxidase catalyzed the converstion of I- to I2 and I2 is extruded into the colloid. The I2 then binds Tg forming MIT and DIT,couping then makes T4 and T3 (2x DIT, MIT +DIT). When stimulated by TSH, the follicular cell takes up iodinated Tg and proteases cleave and release T3 and T4.

Front 2

why is the response to T4 so long?

Back 2

extensive binding to plasma proteins leads to a long half life and a prolonged response (note almost 30 days of elevated BMR in response to injected T4)

Front 3

Describe the effects on development of neonatal hypothyroidism (cretininsm) which can be caused by a lingual thyroid

Back 3

Underdeveloped bone and heigh, extremely delayed mental age

Front 4

Describe target cell response to thyroid hormone (signal transduction)

Back 4

T4 and T3 readily diffusee into the cell, much of T4 is deiondinated to T3. T3 interacts with the thyroid hormone receptor which is bound as a heterodimer with an RXR. THis complex activates the thyroid resposne element of the gene and modulates transcrption

Front 5

what is the effect of thyroid hormone on the sympathetic adrenergic system

Back 5

increase sensitivity to catecholamines due to increased B adrenergic receptor activity, increase receptor number in the heart , liver, muscle, adipoyctes,  (note use of B blockers for Tx of hyperthyrodism)

Front 6

what is the role of TRH in the control of thyroid function

Back 6

acts as a set point determiner (thermostat),

Front 7

what are two possible causes of hypothyrodism

Back 7

1. Primary (low fT4, high TSH, the problem is with the thyroid, e.g. Hashimoto's thryoiditis) 2. Secondary (low fT4, TSH not elevated or inapropriately low, problem is with pituitary)

Front 8

What is the usual cause of hyperthryodism

Back 8

ALmost always primary (high fT4, low TSH e.g. grave's disease autoimmune stimulation of thyroid gland)

Front 9

Describe Grave's disease

Back 9

autoimmune stimulation of the tyroid gland, production of TSI which binds the TSH receptor and triggers the release of thyroid hormone. Find high fT4 but low TSH

Front 10

Goiter is not a disease but rather a sxs. describe how the enlarged thyroid could occur in either hypo or hyper thyrodism

Back 10

1. Hypo-Iodine deficiency leads to low T4, triggers release of TSH to try to elevate levels, TSH induces thyroid hypertrophy 2. Hyper-TSI from graves disease stimulates thyroid gland independent of TSH, hypertrophy and high T4

Front 11

What are the expected levels of Total T4, TBG, free T4 and TSH in  primary hyperthyrodism

Back 11

1. Total T4 is high because the thyroid is secreting too much 2. TBG is normal because the disease does not change this 3. Free T4 is high because the thyroid is secreting too much 4. TSH is low because of negative feedback

Front 12

What are the expected levels of Total T4, TBG, free T4 and TSH in  primary and secondary hypothyrodism

Back 12

1. In both cases total T4 and free T4 are low 2. TBG is unaffected. 3. TSH will be high if the defect is primary because there is no negative feedback because the thyroid gland is not working, TSH will be low if the disease is secondary because there is a problem with the pituitary/ TSH secretion

Front 13

What are the expected levels of total T4, TBG, free T4, and TSH during pregnancy

Back 13

During pregnancy, estrogen stimulates the liver to make more TBG, there will be increase total T4 because of increased capacity but free T4 will be normal because regulation has not changed. TSH will also be normal because there are no problems with regulation. In prenancy must measure free T4 and TSH to get accuate idea of thyroid function.