Endocrine System - Thyroid and Parathyroid Flashcards
Describe the hypothalamic-pituitary-thyroid axis and its role in control of thyroid gland function.
The hypothalamus and pituitary regulate circulating levels of thyroid hormones
Stress decreases thyroid hormone secretion by the gland
Cold increases thyroid hormone secretion
T3 and T4 increase BMR secretion, protein synthesis and sympathetic tone
Thyroid hormone exerts genomic and non-genomic effects.
What is the role of calcium in the body?
Cell division
Cell adhesion
Plasma membrane integrity
Neuronal excitability
Cofactor in coagulation
Muscle contraction
What is Hypothyroidism?
Hypothyroidism is characterized by an underactive thyroid gland, which fails to produce a sufficient amount of thyroid hormones
Caused by - autoimmune thyroiditis, iodine deficiency
Symptoms - fatigue, weakness cold sensitivity, consitpation, irregular menstrual cycles.
What is thyrotoxicosis?
Graves disease produces antibody cells which stimulate thyroid gland
Produces excess T3 and T4 hormone causing hyperthyroidism
Effects - Weight loss despite increased appetite
Rapid or irregular heartbeat (palpitations)
Heat intolerance and excessive sweating
Fatigue and weakness
Tremors or shaky hands
Anxiety, irritability, or nervousness
Very low TSH via feedback to the pituitary
List the principal thyroid hormones.
Basic ingredients for thyroid hormone synthesis are tyrosine and iodine
Iodine + tyrosine -> moniodotyrosine (no endocrine function)
MIT + iodine -> diiodotyrosine (no endocrine function)
DIT + DIT -> thyroxine (T4)
DIT + MIT -> triiodothyronine (T3)
Describe the abnormalities of thyroid function.
Primary hypothyroidism - failure of the thyroid gland to respond to TSH
Causes of primary hypothyroidism:
Thryoiditis
Severe iodine deficiency
Severe deficiency of one or more synthesis enzymes
Removal or destruction of thyroid gland
This would result in high TSH levels as reduced negative feedback and also an enlarged thyroid gland (goitre formation)
Secondary hypothyroidism - deficient TSH production resulting in thyroid atrophy
Tertiary hypothyroidism - deficient TSH secretion due to deficient TRH secretion
How is calcium regulated?
Parathyroid hormone
Activated vitamin D
Calcitonin
How does parathyroid hormone regulate calcium?
When calcium levels drop in ECF, PTH is secreted
In the bone this causes an increase in calcium pumping to ECF and bone resorption
In kidney, PTH activates vitamin D and also there is an increase in calcium reabsorption and phosphate excretion
And in small intestine increase in calcium absorption
How does Active Vitamin D ( 1,25-dihydroxycholecalciferol) regulate calcium?
Regulator of active form is parathyroid hormone
Actions
In the intestine - enhances Ca2+ absorption
Kidneys - facilitates Ca2+ absorption
Bone - increases calcification and mineralisation
How does calcitonin regulate calcium?
Regulator - high Ca 2+
Actions
In bone - inhibits resorption
Kidneys - increases Ca 2+ excretion
List the functional calcium pools in the body.
Bone tissue
extracellular fluid
intracellular calcium
