REGULATION OF SYNTHESIS OF THYROID HORMONES :
* Thyroid hormone releasing hormone ( THRH ) , is secreted by the paraventricular nucleus present in hypothalamus .
* THRH reaches the anterior pituitary by hypothalamo hypophyseal portal system .
* THRH acts on thyrotrophs present in anterior pituitary and stimulate them , to release Thyroid Stimulating Hormone ( TSH ) .
* Thyroid stimulating hormone acts on follicular cells in the thyroid gland and increases the synthesis and release of T3 and T4 hormones .
MECHANISM BY WHICH TSH ACTS ON FOLLICUALR CELLS :
* Follicular cells have TSH receptor on their cell surface .
* This receptor is intracellularly coupled with G stimulatory protein .
* When TSH binds at this receptor , then G Stimulatory get stimulated and eventually activates protein kinase- A .
* Protein kinase A helps with phosphorylation of many proteins
like iodide pumps , thyroid peroxidase etc...
* This increases synthesis and release of thyroid hormones .
GENERAL EFFECTS OF THYROID HORMONE ON OUR BODY :
* T3 and T4 when present, increases basal metabolic rate .
* In absence of T3 and T4 basal metabolic rate falls half of its normal value .
* T3 ,T4 are primarily required for growth and mental development .
EFFECT OF THYROID HORMONES ON CARBOHYDRATE METABOLISM :
* In the presence of T3 and T4 the motility of GIT increases .
* T3 and T4 also stimulates hunger center in hypothalamus .
* GIT absorbs glucose more efficiently in the presence of T3 and T4 .
* Under the influence of T3 and T4 utilisation of glucose by cells increases .
EFFECTS OF THYROID HORMONE ON FAT METABOLISM :
* Thyroid hormones act on adipocytes and increases lipolysis .
* Free fatty acids produced are used for glucose synthesis by process called gluconeogenesis .
* Thyroid hormones decrease cholesterol level in the blood .
EFFECT OF THYROID HORMONES ON HEART :
* Thyroid hormone generally increases the expression of Beta 1 adrenergic receptors .
* So in the presence of T3 and T4 , sympathetic stimulation of heart increases.
* Which leads to increased cardiac output and increased heart rate.
HYPERTHYROIDISM :
* The main causes of hyperthyroidism includes Graves Disease , Thyroid Adenoma
GRAVES DISEASE :
* Graves Disease is an autoimmune disease , where antibodies are produced which over stimulate TSH receptors on follicular cells .
* Which leads to overproduction of T3 and T4 .
* Features include
* BMR metabolic rate increases so much .
* We can even see characteristic bulging of eye ball forwards known as exophthalmos.
* Thyroid gland enlarges leading to goitre .
Clinical features of hyperthyroidism :
1 ) Increased BMR
2 ) Insomnia
3 ) Exophthalmos (mainly in graves disease )
4 ) Tremors
5 ) Intolerant to heat
6 ) Significant weight loss
7 ) Loss of sexual characters and infertility
8 ) Irregular menstrual cycle in females
HYPOTHYROIDISM :
* This condition results when their is chronic deficiency of thyroid hormones
* Main causes of hypothyroidism may be due to defects in thyroid gland itself ( primary hypothyroidism )
Clinical features of hypothyroidism
1) Decreased BMR
2 ) Insomnolence
3 ) Intolerant to cold
4 ) Significant weight gain
5 ) Generalized edema
6 ) Loss of of sexual characters and infertility
7 ) Irregular menstrual cycle in females
8 ) Hair become thick
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