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I don't understand the last part of this question stem though... if the mother's TSH *increases* during pregnancy? Wouldn't this further increase her (and/or the fetus's) production of T4 and thus counteract the hypothyroidism?
@neonem no. Autoimmune hypothyroidism is a destruction of the thyroid gland, and a decrease in production of T3/T4.
An increase in TSH means that there is not enough T3/T4 to inhibit TRH, and so TSH is being released to stimulate the thyroid gland.
TSH, T3, T4 and thyroglobulin cannot cross the placental barrier.
@arezpr although those hormones can't cross, the autoantibodies from Hashimoto's can
The baby has its own TSH though
TSH comes from the pituitary, and act on the thyroid. Autoantibodies attack the thyroid, so TSH doesn't work.