The thyroid produces two thyroid hormones, triiodothyronine (T3) and thyroxine (T4). Thyroid hormone production is regulated by thyroid-stimulating hormone (TSH), which is made by the pituitary gland. When thyroid hormone levels in the blood are low, the pituitary releases more TSH. When thyroid hormone levels are high, the pituitary responds by decreasing TSH production.
Most T3 and T4 circulate in an inactive protein bound form. Only about 0.4% of total plasma T3 and about 0.04% of total plasma T4 circulate in the unbound "Free" and active form. The protein bound hormones serves as a large circulating reservoir which helps buffer any changes in acute thyroid function. Protein bound T3 and T4 are larger molecules and therefore more resistant to excretion in the urine.•
Thyroid hormones are essential to normal development and metabolic homeostasis of the mother and developing child. T3 &T4 generate their effect by activating thyroid hormone receptors (TRs). Activated TRs bind to genes influencing transcription of proteins. Thyroid hormones also increase the number of mitochondria and their activity which results in a rise in O2 consumption, energy liberation and heat production. T3 also influence the effect of insulin, glucagon, growth hormone and epinephrine.•
During the first trimester of pregnancy, increased hCG can alter normal thyroid regulation. HCG increases the synthesis of T3 and T4 which can reduce TSH synthesis by the pituitary. After the first trimester HCG drops and TSH should return to normal.
"Estrogen increases the amount of thyroid hormone binding proteins in the serum which increases the total thyroid hormone levels in the blood since >99% of the thyroid hormones in the blood are bound to these proteins. However, measurements of “Free” hormone (that not bound to protein, representing the active form of the hormone) usually remain normal. The thyroid is functioning normally if the TSH, Free T4 and Free T3 are all normal throughout pregnancy."•
Thyroid dysfunction during pregnancy
- Hyperthyroidism in pregnancy is usually caused by Graves’ disease. Graves’ disease is an autoimmune disorder that results in the production of an antibody called thyroid-stimulating immunoglobulin (TSI), sometimes called TSH receptor antibody, which mimics TSH and causes the thyroid to make too much thyroid hormone. In some people with Graves’ disease, this antibody is also associated with eye problems such as irritation, bulging, and puffiness. Uncontrolled hyperthyroidism during pregnancy can lead to:
* congestive heart failure
preeclampsia—a dangerous rise in blood pressure in late pregnancy
thyroid storm—a sudden, severe worsening of symptoms
low birth weight
Although Graves’ disease may first appear during pregnancy, a woman with preexisting Graves’ disease could actually see an improvement in her symptoms in her second and third trimesters. Remission—a disappearance of signs and symptoms—of Graves’ disease in later pregnancy may result from the general suppression of the immune system that occurs during pregnancy. The disease usually worsens again in the first few months after delivery.•
Treatment of hyperthyroidism during pregnancy is often deferred if it is mild. Severe hyperthyroidism may be treated with antithyroid medications. Propylthiouracil (PTU), may be prescribed for women in the first trimester of pregnancy and then switch to methimazole for the second and third trimesters.
- Hypothyroidism in pregnancy is usually caused by Hashimoto’s disease. Hashimoto’s disease is a form of chronic inflammation of the thyroid gland. Like Graves’ disease, Hashimoto’s disease is an autoimmune disorder. In Hashimoto’s disease, the immune system attacks the thyroid, causing inflammation and interfering with its ability to produce thyroid hormones. Hypothyroidism in pregnancy can also result from existing hypothyroidism that is inadequately treated or from prior destruction or removal of the thyroid as a treatment for hyperthyroidism.
Some of the same problems caused by hyperthyroidism can occur with hypothyroidism. Uncontrolled hypothyroidism during pregnancy can lead to:
anemia—too few red blood cells in the body, which prevents the body from getting enough oxygen
low birth weight
congestive heart failure
* fetal growth delay and developmental defects of the nervous system can result from uncontrolled hypothyroidism during the 1st trimester.
"Hypothyroidism is treated with synthetic thyroid hormone called thyroxine—a medication which is identical to the T4 made by the thyroid. Women with preexisting hypothyroidism will need to increase their prepregnancy dose of thyroxine to maintain normal thyroid function. Thyroid function should be checked every 6 to 8 weeks during pregnancy. Synthetic thyroxine is safe and necessary for the well-being of the fetus if the mother has hypothyroidism."•
Calcitonin is another hormone
produced by the thyroid cells. The calcitonin-producing cells
monitor the calcium level in the blood. If the blood calcium
level rises, calcitonin is produced. Calcitonin then acts on bone
cells to absorb calcium from the blood and store it in bone. It also cause kidney cells to increase the excretion of calcium.
Calcitonin levels rise during pregnancy. The
increase is stimulated by increased serum calcium influenced
by estrogen and hPL. The increased calcitonin inhibits calcium
and phosphorus release from the bones, helping to conserve the
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