Thyroid Hormones

TSH is most sensitive thyroid function test.

 Nonspecific laboratory findings associated with hyperthyroidism

Ø  Decreased serum cholesterol, LDL-C & TAG.

Ø  Increased CPK (due to myopathy)

Ø  Hepatic dysfunction, e.g. raised ALT and ALP

TSH, also known as thyrotropin, is an anterior pituitary hormone; it is a glycoprotein with a molecular weight of about 28,000. This hormone increases secretion of thyroxine and triiodothyronine by the thyroid gland. It has the following specific effects on the thyroid gland:
1. Increased proteolysis of thyroglobulin that has already been stored in the follicles, releasing the thyroid hormones into the circulating blood and diminishing the follicular substance
2. Increased activity of the iodide pump, which increases the rate of "iodide trapping" in the glandular cells, sometimes increasing the ratio of intracellular to extracellular iodide concentration in the glandu- lar substance to as much as eight times normal
3. Increased iodination of tyrosine to form the thyroid hormones
4. Increased size and increased secretory activity of the thyroid cells
5. Increased number of thyroid cells plus a change from cuboidal to columnar cells and much infolding of the thyroid epithelium into the follicles
In summary, TSH increases all the known secretory activities of the thyroid glandular cells.

The most important early effect after administration of TSH is to initiate proteolysis of thyroglobulin, which causes release of thyroxine and triiodothyronine into the blood within 30 minutes. The other effects require hours or even days and weeks to develop fully.

Both T4 & T3 are mostly bound to thyroxine binding Globulin (TBG)

Myxedema can be diagnosed over telephone by hearing frog like husky voice.

Others are secreted from follicular cell

Calcitonin is secreted from parafollicular cell

Graves’ disease
Definition: It is an autoimmune disease in which circulating antibodies are formed against the TSH receptor and activates the thyroid gland hyperactive, resulting in development of goiter and hypersecretion of T3 & T4
More common in women

Clinical features:
1.  Goiter with bruit
2.  Exophthalmos-protrusion of the eye ball (it is the hallmark of Graves' disease)
3.  Retraction of upper eye lid, lid lag
4.  Corneal ulceration
5.  Diplopia
6.  Ophthalmoplegia
7.  Papilledema
8.  Loss of visual acuity
9.  Vitiligo
10.  Digital clubbing
11.  Pretibial myxedema

 

  Nonspecific laboratory findings associated with hypothyroidism.
Ø  Increased serum total cholesterol and LDL-C

Ø  Increased TAG (Triacyl glycerol)

Ø  Hyponatremia (due to free water retention)

Ø  Increased CPK (due to myopathy)

Steroid & thyroid hormones arr lipophilic

TSH works on iodide trapping

Receptors of calcitonin are found in bone & kidney.

Thyroid hormone increase gut motility. So hypothyroid state causes constipation.

Thyroid hormone, an amine, acts on its target tissues by a steroid hormone mechanism, inducing the synthesis of new proteins. The action of antidiuretic hormone (ADH) on the collecting duct (V2 receptors) is mediated by cyclic adenosine monophosphate (cAMP), although the other action of ADH (vascular smooth muscle, V1 receptors) is mediated by inositol 1,4,5-triphosphate (IP3). Parathyroid hormone (PTH), β1-agonists, and glucagon all act through cAMP mechanisms of action.

Explanation-
★Thyroid-stimulating hormone (TSH) is secreted by the anterior pituitary. Dopamine, growth hormone–releasing hormone (GHRH), somatostatin, and gonadotropin-releasing hormone (GnRH) all are secreted by the hypothalamus. Oxytocin is secreted by the posterior pituitary. Testosterone is secreted by the testes.
Reference-Guyton & Hall/13rd/P-927 & BRS physiology/6th/P-233/T-7.1