Thyroid function tests and thyroid physiology

Question 1

Thyroid hormone synthesis 1

Answer 1

• Thyroid makes both T4 and T3 (but mostly T4) in response to increased levels of TSH
• Iodine is actively transported across the basolateral membrane of follicular cells and across the apical membrane to the colloid, where thyroglobulin is stored
• Thyroglobulin (Tg) is a large glycoprotein that will be iodinated and eventually cut up into smaller bits, some of which are T3 and T4
• Tg is iodinated at the apical membrane of the follicular cells, some are tyrosine residues iodinated 2 times (DIT) and some 1 time (MIT)
• Tyrosine residues are iodinated by TPO

Question 2

Thyroid hormone synthesis 2

Answer 2

• To make T3 and T4 DITs are added to either another DIT (T4) or an MIT (T3), this is also done by TPO
• Once T3 and T4 have been produced on the Tg, the Tg molecules are endocytosed into the cell and fuse w/ lysosomes
• This breaks up the Tg into pieces, specifically T4 and T3, and releases the hormones from the basolateral surface for them to enter the blood stream
• There is a 5’ deiodinase nz that is in the cell, activation of this nz causes deiodination of T4 converting it to T3
• Thus activation of 5’D leads to preferential secretion of T3 over T4

Question 3

Metabolism of thyroid hormones

Answer 3

• The main hormone in circulation is T4 (T1/2 = 7 days), which is converted to the more active form T3 (T1/2 = 1 day) in peripheral tissues by type 2 5’D
• T4 can also be inactivated to rT3 (T1/2 = 4 hrs) by 3 5D
• rT3 has no activity, and is one of the two inactive forms of thyroid hormone
• The other inactive form, T2, is from 3 5D (and 1 5’D) inactivation of T3
• Similarly, rT3 can be deiodinated to T2 via 1 5’D
• The inactivated forms and the iodine are both recycled
• All of these nzs require selenium
• Activating nz: 2 5’D makes T3 from T4 (2 5’D not inhibited by PTU)
• Inactivating nz: 3 5D (not inhibited by PTU) makes rT3 from T4, and T2 from T3
• Inactivating (mostly) nz: 1 5’D (is inhibited by PTU) makes T2 from rT3 and T3

Question 4

Circulation of thyroid hormone 1

Answer 4

• 80% of thyroid hormone circulated bound to thyroxine binding globulin (TBG)
• 15% is bound by TTR (transthyretin) and 5% is bound by albumin
• TBG is most important, as it has 100x the affinity of TTR for T4
• TBG levels may very, and they are increased during pregnancy or OCPs (high estrogen states)

Question 5

Circulation of thyroid hormone 2

Answer 5

• As long as the person is euthyroid, the free T4 will be normal but the total T4 depends on the levels of TBG
• Thus a women taking OCPs will have a high total T4 but normal free T4 (assuming euthyroid)
• Total T4 will change in the same direction of TBG change to ensure free T4 is in normal range
• Other things that can raise TBG levels: liver disease, HIV, drugs (5FU, heroin)
• Things that can lower TBG levels: non-thyroidal illness, nephrotic syndrome, GCCs, androgens/steroids

Question 6

Determining free T4 1

Answer 6

• Can directly and indirectly measure free T4 levels
• Indirect measurements: 1) give radioactive T4 and measure its uptake w/ TBG (TU) and 2) give radioactive T3 and a resin and measure the T3 uptake w/ resin
• For 1) the uptake will be proportional to the TBG concentration (more TBG = more T4 uptake) and for 2) the TBG concentration is inversely proportional to the resin uptake (less TBG means more resin uptake)

Question 7

Determining free T4 2

Answer 7

• For 1), once the TU is calculated the free T4 can be calculated by dividing the Total T4 (TT4) by TU (FT4 = TT4/TU)
• For 2), FT4 = TT4 x T3U
• Overall: compare TT4 to TBG concentration, if TT4 is nl and TBG is high-> FT3 is low
• If TT4 changes in the same direction as TBG-> FT4 is nl (this is normal response)
• But now the direct measurement is used

Question 8

TSH/FT4 relationship

Answer 8

• Small changes in T4 lead to large changes in TSH (inversely related), thus can detect diseases by monitoring changes in TSH rather than T4
• Everyone has their own set point at which normal TSH/T4 levels are needed to maintain euthyroid status
• Limitations of TSH to assess thyroid status: Rx of thyroid disease, pituitary disease, non thyroidal illness

Question 9

TSH lag phase

Answer 9

• After Rxing hyper/hypothyroidism there is a lag phase in normalization of TSH levels
• It takes 12-16 wks for TSH levels to normalize, but only 8 wks for T4 levels to normalize
• This is b/c the thyrotroph cells (releasing TSH) are either hypertrophied (in hypo) or atrophied (in hyper) due to the T4 deficiency/excess
• It takes extra time for these cells to regain normal function and secrete TSH at an appropriate level

Question 10

Mechanism of action of thyroid hormone

Answer 10

• Specific thyroid hormone transporters allow T4/T3 to cross the membrane
• In the cell T4 is converted to T3 using 2 5’D
• T3 binds to its receptor in the nucleus, causing either decreased or increased transcription of genes
• In peripheral tissues, T3 binding to its receptor causes increased gene transcription and regulation of growth, differentiation and calorigenesis
• In the pituitary/hypothalamus, T3 binds to its receptor and causes inhibition of transcription leading to decreased TSH/TRH production/release
• Important to note: T4 is the main feedback regulator on pituitary and hypothalamus (i.e. increase in T4 serum levels, not T3, will cause decreased TSH/TRH), but this T4 is converted to T3 inside the cells in order to have any action in lowering the TSH/TRH levels

Question 11

Non-thyroidal illness (NTI)

Answer 11

• Changes in serum T3/T4 levels are seen when under non-thyroidal illness or caloric deprivation
• As the illness/fasting progresses T3/T4 levels lower slowly until the illness becomes severe, at which point the T3/T4 levels are lowest
• Simultaneously the rT3 levels are reciprocal to t4/T3, and thus are elevated most at maximally ill
• Mortality mirrors rT3 levels

Question 12

Low T3 vs low T3/T4 states 1

Answer 12

• In NTI, there are 2 distinct phases
• First there is a low T3 state, where only T3 is low and rT3 is high
• T4 is normal and TSH is normal
• As the illness progresses and the pts becomes more sick, TT4 drops and TSH becomes variable (can be low or high)

Question 13

Low T3 vs low T3/T4 states 2

Answer 13

• Reasons for these changes: there is an inhibition of 5’D nzs (1 and 2 5’D)
• This means T4 can’t be converted to T3 as well (uses 2 5’D) and rT3 can’t be converted to T2 as well (uses 1 5’D)
• Thus more T4 is shunted to rT3 (uses 3 5D) and the rT3 isn’t broken down
• This also results in elevation of other thyroid hormone breakdown products (T3S and T3AC)
• These nzs change b/c their activity is controlled by cytokines and nutrition/metabolism

Question 14

Low T3 vs low T3/T4 states 3

Answer 14

• As the illness progresses and pt enters low T3/T4 state, the T4 levels fall due to loss of TBG-T4 binding (change in TBG conformation and TBG inhibitor)
• The inhibitor is the same that inhibits 5’ D nzs, the resultant free T4 is metabolized
• WBCs that are activated during the illness use TBG/T4 complexes as an iodine to generate ROS
• This further reduces T4 levels, since the WBCs breakdown the T4 to isolate the iodine
• Overall changes in low T3/T4 state: high rT3 due to inability of rT3 breakdown (inhibition of 1 5’D), low T3 due to low T4->T3 conversion (tissue dependent) plus the T3 that is there is more quickly converted to T3S and T3AC, and low T4 due to inhibitor of TBG (FT4 then metabolized) and WBC breakdown of TBG/T4

Question 15

Regulation of TSH during NTI

Answer 15

• High cytokine state leads to increased T3 inside the hypothalamus
• This is b/c in the hypothalamus there is increase 2 5’D activity and reduced 3 5D (this is opposite to most other tissues in the body during this state)
• This results in increased T3 levels (increased production and reduced breakdown) inside the hypothalamus PVN and thus lower TRH generation
• The low TRH state results in low TSH release from pituitary

Question 16

Anti-TPO Ab

Answer 16

• Ab against TPO is marker of autoimmune dysfxn, and predicts the etiology/progression of thyroid disease
• Will be positive in graves disease (hyperthyroidism-> low TSH) and positive in hashimoto’s disease (hypothyroidism-> high TSH)