Thyroid function Flashcards
What is the thyroid gland and what is it responsible for?
- Butterfly shape organ located in neck – near the Larynx.
- Responsible for the secretion of thyroid hormones ( T3 and T4 )
What is the parathyroid gland and what is it responsible for?
– 4 yellowish organ located w/in the thyroid gland – primary responsibility in maintaining calcium levels
What is thyroglobulin?
main storage site of thyroid hormones. Is the protein precursor to thyroid hormones
What is thyrotropin (TSH)
regulated thyroid hormones
What is T3 and what does it do?
- triiodothyronine - Regulates metabolism, growth and development
- 20% of T3 derived from the thyroid gland
- Most come from de-iodinzation of T4 from liver, kidney and muscle
What is T4 and what does it do?
- thyroxine- major hormone secreted from the thyroid gland
- Most is bound by TBG - FT4 - Only the unbound fraction is biologically active
- Other biologically inactive forms – rT3, MIT, DIT
- MIT and DIT are precursors to T3 and T4
- rT3 comes from peripheral de-iodination of T4
What is calcitonin and what does it do?
– Not much known about physiologic role in humans
- Release is stimulated by increasing circulating calcium levels in blood
- Serves as a antagonist to PTH – inhibits osteoclastic bone activity ( decrease calcium )
How is thyroid hormone synthesized in the thyroid gland?
- Iodine is ingested in food and water, concentrated in thyroid gland and” trapped”
2.Incorporated into the amino acid tyrosine – ( Thyroglobulin )
3.Concentrated iodine is oxidized and bound to tyrosyl residues on thyroglobulin – catalyzed by thyroid peroxidase ( TPO )
4.MIT ( monoiodothyronine ) and DIT ( Diiodothyronine ) are formed as a result
TPO acts to couple the MIT and DIT to form the T3 and T4 ( active forms of thyroid hormone )
5.TSH stimulates lysosomes to cleave T3 and T4 and release into blood stream
Up to 80% of T4 is further Deiodinated to T3 and rT3 in the liver
T3 is metabolically active, rT3 is not ( when T3 is up, rT3 is down )
What carrier proteins is thyroid hormone bound to?
- Thyroid binding protein binds as much as 80% of thyroid hormones
- Thyroid binding prealbumin
- Albumin
- Transthyretin
- The remaining amount ( <1/2 % ) of T3 and T4 are free form and metabolically active – free to travel across cell membranes
- Amount of circulating total T3 and Total T4 are affected by circulating binding
How does the Hypothalamus-Pituitary-Thyroid Axis – control mechanism for thyroid function?
- Operates on neg feedback
1. Hypothalamus – releases TRH ( thyroid stimulating hormone ) – in repsonse to circulating throid hormone
2. Pituitary – releases TSH in response to TRH secretion
3. Thyroid hormone – Release T3 and T4 in response to TSH secretion
4. Circulating T3 and T4 controls release of TSH ( short loop ) and TRH ( long loop )
5. T3 and T4 perform many function – act on liver, heart, bone and CNS
How is TSH released?
released in pulsating nature and is diurnal
When is TSH highest?
highest at night
How long is the half life is TSH?
- half-life is relatively long (50 min 0 compared to other hormones
- Single measurement is adequate for assessing circulating levels
- This is why it is a good biomarker for hypo and hyperthyroidism
What are the three generations of TSH testing?
1st generation – RIA
2nd generation – Immunoradiometric
3rd generation – Chemilluminescence
In what conditions is TSH commonly increased or decreased in?
Best initial test for use in ambulatory, non-hospitalized patients
Increased in hypothyroidism
Decreased in hyperthyroidism
What is FT4 used for?
used in conjunction with TSH ( normal – 0.7-1.8 mg/dL )
When would you see an decrease in FT4?
FT4 - decreased in hypothyroidism
when would you see an increase in FT4?
FT4 – increased in hyperthyroidism
