Thyroid hormonesThyroid hormones

Hormones

Thyroid gland

Thyroid gland secretes 3 main hormones

• Thyroxine (T4)• Triiodothyronine (T3)

• Calcitonin

Energy &

Growth

Control of calcium

Hormones- Regulation of TH secretion:

Thyroid Gland

TH

TSH

Anterior Pituitary

Hypothalamus

TRH--

--

Hormones

Calorigenic actions BMR (stimulation of oxygen consumption by tissues)

Adipose tissues (catabolic lipolysis ) and cholesterol

Muscle (catabolic protein breakdown)

Body temperature ( heat production 2ry to energy production)

Bone, skeletal muscle and nervous system (normal development).

Actions of thyroid hormoneActions of thyroid hormone

Hormones

Heart (upregulation of β receptor and sensitivity to circulating catecholamines).

CNS stimulation resulting in anxiety, restlessness, insomnia and tremors.

Actions of thyroid hormoneActions of thyroid hormone

All these actions are remarkable in patients with abnormally hyperthyroidism

Hormones

Thyroid hormone blood tests 1-Total thyroxine (T4)

T4 in the blood is attached to a protein called the thyroxine-binding globulin.. A total T4 blood test measures both bound and free thyroxine.. (Normal values: T4 = 4.5–12.0 µg/dL

2- Free Thyroxine (FT4) Measures only free thyroxine, as free thyroxine

affects tissue function in the body. (Normal values: FT4 8–2.4 ng/dL)

Hormones3-Total T3 (triiodo-L-thyronine):

A total T3 blood test measures both bound and free T3. (Normal values: T3 = 80–200 ng/dL)

4- Free T3 (FT3): Measures only free T3

(Normal values: FT3 0.2–0.6 ng/dL)

NB: T3 is quicker in action (reaches its peak activity

faster) and is of shorter t ½ life than T4. Generally, T3 is about five times as potent as T4 why ?.

T3 is loosely bound to TBG than T4, more available in free active form

T3 binds to TH receptor with more affinity than T4

Hormones

5- Thyroid stimulating hormone (TSH): TSH levels is the most sensitive and

specific test for thyroid dysfunction (why?).

Any small changes in free TH level produces exponential or logarithmic changes in TSH

(Normal level in body is 0.2 – 6 mU/L)

NB: a raised level of TSH means 1ry hypothyroidism while a lowered level means 1ry hyperthyroidism

Hormones

6- Other blood tests6- Other blood tests

a- Thyroid antibodies test:a- Thyroid antibodies test: This test measures the presence of Abs against thyroid tissue. Antibodies may mean that you have an autoimmune disease such as Hashimoto’s thyroiditis (hypothyroidism) or Graves' disease (hyperthyroidism) !

b- Thyroxine-binding globulin (TBG) b- Thyroxine-binding globulin (TBG) test. test. TBG is an important protein in the blood that carries the thyroid hormones T3 and T4. TBG testing is not done very often.

Hormones

Thyroid hormones and lab. values

Hypo- Hyper-

TSH ↑ ↓Total T4 & T3 ↓ ↑FT3 & FT4 ↓ ↑Total cholesterol

↑ ↓

LDL ↑ ↓HDL ↓ ↑

Hormones

HyperthyroidismThe thyroid gland excessively

Overproducing thyroid hormone

Toxic goiter (Graves’ disease)

Autoantibodies(TSI)

TSH Receptor on Thyroid

Gland

Excessive TH production

Hormones

Hormones

Symptoms

•Protrusion of the eye ball (exophthamlus), goiter and HR

•Heat intolerance, weight loss, excessive sweating and GIT motility

•Nervousness, irritability, restlessness and muscle weakness

Hormones

TreatmentAntithyroid drugs: thioureylenes propylthiouracil, methimazole & carbimazole

Surgery ( may cause hypothyroidism or hypoparathyrodism)

Radioactive I131 (may cause delayed hypothyroidism).

Hormones

Other causes of hyperthyroidism

• Hyperfunctioning follicular adenoma in thyroid gland (TH TSH) (1ry Hyperthyroidism)

• 2ry hyperthyroidism (pituitary gland) ( TSH TH)

• 3ry hyperthyroidism (hypothalamus) ( TRH TSH TH)

Note the difference in lab values

Hormones

Hypothyroidism

A clinical syndrome in which the deficiencyor absence of thyroid hormone slows body metabolic processes.

Hormones

1- Cretinism1- Cretinism

Hypothyroidism in childrenIf untreated, it results in mild to severe impairment of both physical and mental growth and development.Symptoms

• Dwarf and obese

• Infertile and mentally retarded

Hormones

Causes•Maternal iodine deficiency•Maternal antithyroid antibodies•Congenital abnormalitiesIf the pregnant woman has abnormal hypothyroid function, this should be corrected during pregnancy or baby will have severe mental deficiency at birth (difficult to be treated)

If the pregnant woman has normal thyroid function, baby will be normal. However, diagnosis should be done early at birth and rapid ttt by TH replacement therapy should be started if hypothyroidism is diagnosed

Hormones2- Hypothyroidism in adult (myxedema):

Symptoms :

• Weight gain, lethargy and sluggishness • Puffiness of skin (myxedema) •Bradycardia, cold intolerance and constipation

Hypothyroid MyopathyWith high serum creatine kinase values

Anemia in hypothyroidism

Normocytic Lack of erythropoietin production arising from the reduction in need of O2.

MicrocyticIron deficiency anemia is related with menorrhagia and iron malabsorption.

MacrocyticFailure of vitamin B12 absorption occurs in pernicious anemia.Impaired intestinal absorption of folic acid.

Hormones

Causes• Mainly Hashimoto’s (1ry hypothyroidism)

Autoantibodies are directed to thyroglobulin or thyroid peroxidase inhibiting TH synthesis in thyroid gland

• Rarely pituitary or hypothalamic hypofunction (2ry or 3ry hypothyroidism)

Treatment: TH replacement therapy

Hormones

3- Simple 3- Simple Goiter:Goiter:Hypothyroidism due to iodine deficiency

synthesis of thyroid synthesis of thyroid hormonehormone TSH level TSH level thyroid thyroid gland hypertrophy.gland hypertrophy.

Causes

•Mainly Mainly diminished intake of food diminished intake of food containing iodinecontaining iodine

•Increased intake of Increased intake of food food containing goitrogenscontaining goitrogens (cabbage) (cabbage)

•DrugsDrugs (lithium) (lithium)

Hormones

Simple Goiter:Simple Goiter:Treatment

Initially Levothyroxin normalization of TSH and Initially Levothyroxin normalization of TSH and gland size.gland size.

Surgery may be needed to normalize gland size. Surgery may be needed to normalize gland size.

Then Iodine supplementation in dietThen Iodine supplementation in diet

as preventive treatment.as preventive treatment.

N.B. Iodine supplementation as Iodine supplementation as initial therapy thyrotoxicosis initial therapy thyrotoxicosis due to TH production due to TH production

Drug-induced thyroid disorder

Iodine-containing drugs (amiodarone)-induced hyperthyrodism

IncidenceIn areas with high iodine intake - 1.7%In areas with low iodine intake - 12%

PathogenesisType 1 (AIT): occurs particularly in patients with underlying thyroid disease; iodine induced excessive synthesis of thyroid hormone.Type 2 (AIT): destructive thyroiditis (amiodarone-induced); occurs in patients with no previous underlying thyroid disease; thyroiditis results in release of thyroid hormones into the circulation.

Serum IL-6 and color flow doppler ultrasound assessment (thyroidal blood flow) are important diagnostic tools in the differentiation between T1AIT and T2AIT.

TreatmentIf possible withdrawal of amiodarone. T1AIT: treatment by Large doses of antithyroid drugs including methimazole or propylthiouracil.T2AIT: steroid treatment.

Amidarone-induced hypothyrodism

Such as in iodine-sufficient parts of the world, such as the united states.

High amount of iodine released during the metabolism

Wollf-Chaikoff effect

Hormones

Drugs inhibit TH synthesis through :

- Competition with iodide for transport to the gland.

-Inhibition of thyroperoxidase.

Drugs decrease peripheral deiodination (T4 to T3).

Levothyroxine is sometimes used with antithyroid drugs in the treatment of hyperthrodism.

Acetaminophen but not aspirin is used to alleviate hyperthermia in hyperthyrodism.

Wolf Chaikoff effect.

Patient on proylthiouracil cannot receive radioactive therapy.

Hormones