Thyroid Hormone & Fertility

My previous post on thyroid health (see my #womenshealthwednesday page) was a brief overview of the multiple functions of thyroid hormone in the body, and general signs and symptoms of thyroid excess and deficiency.

Today I want to talk about the importance of optimal thyroid function with respect to fertility.

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If you are HYPOthyroid, meaning you are not producing adequate amounts of thyroid, or your body isn’t converting T4 into T3 (the more active form of thyroid) will significantly impact the LUTEAL phase of the menstrual cycle.

Low levels of peripheral thyroid will signal to the hypothalamus to release Thyrotropin-releasing hormone (TRH). The increase in TRH from the hypothalamus will stimulate the pituitary gland to release Thyroid Stimulating Hormone (TSH) which in turn stimulates the thyroid to release T4 and T3 (your thyroid hormones). However, if your body isn’t making adequate amounts of active thyroid hormone, this feedback loop of inadequate levels of thyroid hormone will continue to stimulate the hypothalamic production of TRH. As well as stimulating TSH production at the level of the pituitary, TRH also stimulates the production and release of prolactin. If prolactin is in excess, it will inhibit Gonadotropin-releasing hormone (GnRH). GnRH is responsible for stimulating the release of Luteinizing Hormone (LH) and Follicle-Stimulating hormone (FSH). LH and FSH are the major hormone players at the level of the ovary to stimulate follicle production and promote ovulation.

Therefore, if your thyroid is under functioning, prolactin can become elevated which will impact follicle development and potentially inhibit ovulation.

Another way a low functioning thyroid will impact your fertility is through the more active thyroid hormone, T3. If ovulation has occurred during your cycle, T3 acts on the luteal cells in the ovary. The luteal cells in the ovaries are where progesterone is synthesized and released into the body. Progesterone, among other functions, is crucial to maintaining the uterine lining for the remainder of the cycle and then through the start of the pregnancy until the placenta takes over. Therefore, if you have an under functioning thyroid, resulting in low T3 production, you may not have enough hormonal stimulation even if you are ovulating, to produce enough progesterone to hold and maintain a healthy pregnancy.

The endometrium and trophoblast (placenta in its early developing stages) both have receptors for TSH and T3, T4. These receptors are present and require adequate amounts of thyroid stimulation in order to promote a successful implantation. Too much thyroid hormone (hyperthyroid) will overstimulate the receptors, and not enough thyroid hormone (hypothyroid) won’t stimulate the receptors enough. Implantation of a blastocyst into the endometrium is a delicate dance of the immune system and hormones therefore it is important to have both of these aspects functioning optimally when gearing up for a healthy pregnancy.

Given the direct impact that thyroid hormone has on fertility, and that it has a hand in most other major organ system functioning, it is important to get your thyroid hormone properly assessed and back in balance to promote overall health and optimize your fertility!

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