thryoid hormones Flashcards
What are the 2 thyroid hormones?
T3 (triiodothyronine), T4 (thyroxine)
Which thyroid hormone is the more active form?
T3
Which hormone acts of the the thyroid gland to trigger T3/T4 release?
Thyroid stimulating hormone (TSH)
What are the steps in the production of the thyroid hormones in the thyroid gland?
- thryoid gland absorbs iodine from the bloodstream
- follicular cells synthesise thyroglobulin which is secreted into the colloid
- iodine is bound to tyrosine residues on thryoglobulin, forming precursors to T3 and T4.
- T3/T4 are formed and stored in thryoid follicles until they are released under the stimulation of TSH
What is the main carrier protein for thyroid hormones?
Thyroxine-binding globulin
What is the structure of T4?
Contains two DITs, which have a total of 4 iodines
What is the structure of T3?
Contains a DIT and MIT, with a total of 3 iodines
What is the relationship between free and bound thryoid hormones in the blood stream?
They are in a dynamic equilibrium. The unbound/free fraction is a small percentage but is physiologically active.
What is the receptor binding of T3/T4?
They enter cells and bind to nuclear receptors to influence gene expression
What are the metabolic functions of thyroid hormones?
Increase basal metabolic rate (raises body temperature), promotes oxygen consumption and energy production
What is the role of thyroid hormones in infants/foetuses?
Nervous system development
What are the negative feedback loops for thyroid hormones?
T3 and T4 have negative feedback to both the hypothalamus and anterior pituitary, reducing TRH and TSH release respectively
What are the causes of hyperthyroidism?
Grave’s disease, toxic nodular goitre, thyroid adenoma, thyroiditis
What is Grave’s disease?
An autoimmune disorder where the thyroid gland is stimulated by an abnormal immunoglobulins that have a TSH-like effect on TSH receptors causing excessive T4/T3 release.
What is toxic multinodular goitre?
Nodules in the thyroid gland become able to synthesise and secrete T3/T4 independent of TSH receptor stimulation
What is a toxic thyroid adenoma?
A benign tumor of the thyroid gland that produces excess T3/T4
What is thyroiditis?
Inflammation of the thyroid gland, leading to the release of stored thyroid hormones into the bloodstream.
What is T3 thyrotoxicosis?
Increased levels of free T3, but normal T4 and low TSH
What are the symptoms of hyperthyroidism?
Weight loss, increased appetite, anxiety, tachycardia/palpitations, arrhythmias, insomnia, fatigue, exophthalmos, goitre, heat intolerance, excessive sweating
What are the results of thyroid function tests in hyperthyroidism?
TSH is usually low, while T3/T4 are elevated
What are the results of thyroid antibody panels in hyperthyroidism?
If caused by Grave’s Disease, will be positive for Thyroid stimulating antibodies
What will be the result of a Radioactive Iodine reuptake test in Grave’s disease?
High, diffuse uptake
What will be the result of a Radioactive Iodine reuptake test in thyroiditis?
Diffuse pattern of low uptake
What will be the result of a Radioactive Iodine reuptake test in toxic multinodular goitre?
Focal uptake in thyroid nodules
What will be the result of a Radioactive Iodine reuptake test in toxic adenoma?
‘Hot’ nodule - area of increased uptake
What investigations can be used to diagnose hyperthyroidism?
thyroid function test, thyroid antibody panel, radioactive iodine reuptake test/scan, ultrasound
What are the non-pharmacological interventions for hyperthyroidism?
full/partial thyroidectomy, Radioactive iodine therapy
What are the pharmacological interventions for hyperthyroidism?
Anti-thyroid drugs (e.g. carbimazole), Beta-blockers for anxiety/arrhythmia
What are the causes of hypothyroidism?
Hashimoto’s thyroiditis, Iodine deficiency, thyroidectomy/radioactive iodine therapy, pituitary dysfunction
What is Hashimoto’s Thyroiditis?
An autoimmune condition that causes the destruction of the thyroid gland
What dietary deficiency is associated with hypothyroidism and goitre?
Iodine
What is primary hypothyroidism?
A condition where the thyroid is unable to produce thyroid hormone, despite high levels of TSH
What is secondary hypothyroidism?
Low thyroid hormone release due to insufficient stimulation from low TSH levels
What are some causes of primary hypothyroidism?
Hashimoto’s thyroiditis, iodine deficiency, thyroidectomy, drugs
What are some causes of secondary hypothyroidism?
Lesion/stroke of pituitary gland destroying TSH secreting cells
What are the symptoms of hypothyroidism?
fatigue, weight gain, reduced appetite, cold intolerance, dry skin, thinning hair, brittle nails, depression, constipation, bradycardia, muscle aches, puffy face, goitre, menstrual irregularities
What are the results of thyroid function tests in hypothyroidism?
T3/T4 are low. TSH is high in primary, and low in secondary hypothyroidism
What are the typical results of a thyroid anti-body panel in hypothyroidism?
If hypothyroidism is caused by Hashimoto’s will likely to positive for anti-thyroid peroxidase antibodies
What is the appearance of a radioactive iodine reputake scan in primary hypothyroidism?
There will be diffuse, low uptake
What is the appearance of a radioactive iodine reputake scan with a non-secreting thyroid nodule?
‘Cold’ nodule - nodule will have less uptake than the rest of the thyroid
What are the treatments for hypothyroidism?
hormone replacement therapy (levothyroxine), iodine supplementation
