Thyroid Gland

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


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 maternal skeleton.


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