Before we dive in, some physiology – What is the role of the thyroid hormones and how this level is regulated?
Thyroid hormones are critical for normal growth and development to embryos life, infancy and childhood. In adults they are responsible, among others, for proper metabolism.
Thyroid hormones T3, T4 are produced and secreted from the thyroid gland located in the neck. T3 is the active hormone, and T4 becomes T3 at the cellular level. These hormones level is regulated by the pituitary gland (in the brain) called TSH. When the level of thyroid hormones increases, the level of TSH decreases, when the level of thyroid hormones decreases (a condition called under-active thyroid), TSH level rises.
It is common to divide hypothyroidism in to two categories:
- Sub-clinical activity – when the level of active hormones T3 and T4 are lower than normal.
- Sub subclinical activity – when the level of active hormones T3 and T4 are fairly low but provides compensation by an increase in TSH values that bring the active hormone levels to a new equilibrium.
Thyroid and Pregnancy
You can look at Pregnancy as a “stress test” for the thyroid gland. Pregnancy significantly affects the thyroid gland and its functions. During pregnancy the gland will grow by 10% in size and thyroid hormones production will increase by about 50%. In the first trimester of pregnancy the fetus still doesn’t have an independent functioning thyroid and therefore depends on his mother for supply of these hormones.
Normal laboratory changes during pregnancy
Due to the increase in hormone levels and due to the influence of the BHCG on the thyroid gland (BHCG – binds to the thyroid gland and causes the production of thyroid hormones) in lab testing terms there is a difference in the “norms” when looking at pregnant woman TSH level values according to the testing laboratory. If there are no laboratory norms the recommendations of the American Association are the norms that will be determined per trimester:
– First trimester normal TSH Levels are 0.1 to 2.5 mIU-L
– Second trimester normal TSH Levels are 0.2 to 3 mIU- L
– Third trimester normal TSH Levels are 0.3 to 2.5 mIU-L
Under-active thyroid gland (hypothyroidism) in pregnant woman
Hypothyroidism while pregnant is divided into two categories:
Clinical hypothyroidism – TSH levels are elevated (compared to the specific trimester of pregnancy) and low levels of FT4 or TSH levels are above 10 mIU-L. Clinical Hypothyroidism is related to increased chances of premature birth, miscarriage and low birth weight, as well as a negative impact on neurocognitive development of the fetus.
Subclinical hypothyroidism – TSH levels are between 2.5 to 10 mIU-L and you have normal FT4 levels. The evidence regarding the negative effects of subclinical hypothyroidism on pregnancy and the developing fetus are less strong than for clinical hypothyroidism.
Best research conducted on the subject divided randomly 4,000 women in early pregnancy stages into two groups: one group underwent tests of thyroid functions and antibody titer, if the levels of TSH were over 2.5 mIU-L and antibodies titer was positive the tested women were treated with thyroxine in order to bring TSH Levels to less than 2.5, the second group did not undergo screening and there for received a conventional pregnancy follow-up. At the end of the last follow-up the first group observed less childbirth complications.
Thyroiditis after birth
Postpartum thyroiditis is a disorder (usually a temporary condition) in thyroid function that develops during the first year after birth in women which had no thyroid disorder pre-pregnancy. Prevalence is about 8% in the general population and increases to 33-50% of women with a positive antibody titer. The symptoms may come from the thyroid: emotional irritability, rapid pulse, intolerance to heat
Or from an under-active thyroid: dry skin, difficulty concentrating, fatigue and intolerance to cold.
There are also studies that tie the postpartum depression and thyroid disorders.
Therefore, for women with a positive antibody titer it is important to monitor thyroid function after birth.