Thyroid Flashcards
What is the general structure of a thyroid hormone (thyroxine) molecule?
An amino acid attached to an inner and an outer ring structure which are connected by an ether link. Each ring holds 2 iodine atoms and the outer ring holds one hydroxy group as well.
How is T4 altered to produce T3 (the active nuclear hormone)?
It is deiodinated.
How can radioactive iodine make its way into the human body?
Ingestion or inhalation. Easily dissolved in water so moves through atmosphere and into organisms freely.
How can radioactive iodine be used in medicine?
Low dose: treat overactive thyroid
High dose: treat thyroid cancer
What are the 4 main effects of thyroid hormone activity?
- Normal fetal/child development (esp. brain)
- Maintain basal metabolic rate
- Manage cardiovascular system
- Modulate other hormones’ functions
(+ many others not required to know!)
Which category of endocrine disease is the most common? Does it affect one gender more strongly than the other?
Thyroid hormone diseases are the most common and disproportionately affect women.
What differentiates hypothyroidism from hyperthyroidism?
Hypothyroidism: low levels of TH
Hyperthyroidism: high levels of TH
(complex connection with estrogen)
What is the current trend for incidences of thyroid disease in North America? What is a possible explanation for this?
Thyroid disease is on the rise, maybe due to increased use of endocrine disrupting compounds and molecules which mimic thyroid hormone structure.
What structures make up the thyroid gland?
Many colloid-filled sacs called follicles which have capillaries to allow thyroid hormone to be released into the blood.
What is the precursor to thyroid hormone? What converts this into thyroxine?
A large protein called thyroglobulin. Converted into thyroxine by thyroid peroxidase.
How does thyroid peroxidase help convert thyroglobulin into thyroxine?
By adding iodine to 2 tyrosine residues on the protein, then connecting them by forming an ether linkage. The protein chain is degraded, releasing only thyroxine into the blood.
What 10 steps outline the synthesis of thyroid hormone to its release into the bloodstream?
- Uptake of iodide (active transport)
- Uptake of amino acids, sugars
- Production of thyroglobulin (TG)
- Packaging of TG in Golgi
- Secretion of TG into colloid
- Add iodine to TG
- Storage in colloid until needed
- Reabsorption of TG
- TG digested
- Thyroxine released into blood
What stimulates the reabsorption of thyroglobulin from the colloid and its conversion into thyroid hormone?
Thyroid stimulating hormone.
How do people in North America consume the majority of the iodide needed for biological function?
As iodized salt (iodide added to salt).
What causes goitre?
Lack of iodine, and therefore no production of thyroid hormone. No negative feedback and so thyroid stimulating hormone keeps upregulating the thyroid gland, causing enlargement.
What occurs to thyroid hormone when there is an excess of iodide?
Thyroid hormone synthesis is downregulated.
What is the name of the process in which an iodine is removed from the outer ring of thyroxine (T4)? What enzyme does this? What is the result?
“Step up” done by 5’-Deiodinase. Results in T3, the most active form of thyroid hormone.
What is the name of the process in which an iodine is removed from the inner ring of thyroxine (T4)? What enzyme does this?
“Step down” done by 5-Deiodinase. Results in reverse T3 (T3r), which is inactive in the nucleus.
What differentiates the effects of T3 from the other thyroid hormones (T4, T3r, etc.)?
T3 is active in the nucleus and can affect transcription, while the other thyroid hormones have mostly non-genomic effects.
Why are deiodinases important for physiological functions?
Allow local tissue modulation of thyroid hormone, so that effects can be altered.
How is thyroxine (T4) metabolized and/or deactivated?
Mostly by deiodination to T3 and T3r, but some is also deactivated in the liver and kidney (sulfation, etc).
What is a thyrotroph?
A small portion of the anterior pituitary cells which release thyroid stimulating hormone.
How are thyrotrophs stimulated to release thyroid stimulating hormone? How are they inhibited?
Stimulated by thyrotropin-releasing hormone and inhibited by somatostatin (both from hypothalamus). Also inhibited by negative feedback from thyroid hormone.
What can cause the hypothalamus to secrete thyrotropin-releasing hormone?
Inputs from the circadian rythm or from environmental factors (exposure to cold).