• Dr Agnes N Ryu

Metabolic Health & Fertility

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Thyroid assessment for pregnancy

Agnes N Ryu

Agnes N Ryu

Pathways Clinic


I find that a high proportion of the patients who come with fertility issues have suboptimal thyroid function and are living with a long term hypometabolic condition.


The thyroid test is essential as part of a preconception assessment for anyone trying to conceive.

If any of following is applicable, a full thyroid assessment should be done as soon as possible.

-It has taken more than six months to conceive.

-There have been any previous miscarriages

-The woman has an irregular menstrual cycle or ovulation problem.

-The woman has polycystic ovarian syndrome.

-There is any family history of thyroid problems.



Many cases of thyroid problems are missed because a comprehensive test has not been performed. The thyroid blood tests should include the following:


-Thyroid-stimulating hormones (TSH): the pituitary hormone sensing the circulating thyroid hormone level

-Thyroxine (T4) – total and free T4: a prohormone that should be converted to T3 to be used in cell

-Triiodothyronine (T3) – total and free T3 : the active thyroid hormone that transports oxygen and energy into cells, regulating metabolism

-Reverse T3 (rT3) – it measures how much free active T3 binds at thyroid receptors and turns them off instead of activating them. RT3 is produced in stressful situations to slow down metabolism to preserve the system.


-Thyroid antibodies: can be evidence of inflammation of the gland or tissue destruction by autoimmunity. TPO Antibodies (Thyroid Peroxidase Antibodies) and TG antibodies (thyroglobulin antibodies).




The TSH range is set statistically, with the lowest 2.5% of readings being defined as hypothyroidism and the top 2.5% being defined as hyperthyroidism. The “normal” range is so wide that there is good reason to question its validity. The TSH reference range in the UK remains the widest in the world (0.5-10 mlU/L) resulting in many people suffering symptoms of hypothyroidism without the benefit of a diagnosis or treatment. Many people with Hashimoto’s autoimmune thyroid disease may have a normal TSH reading so this test alone cannot rule out the presence of thyroid disease.

The TSH range for fertility differs from that for the general population. Ideally TSH levels should be monitored and lowered prior to fertility treatment and conception to reduce the risk of harm to the development of the baby and complications during the pregnancy.

It is suggested that for optimal fertility, TSH should be at the lower end of the reference range, in the 1.0 – 2.0 mIU/ L range. In general, TSH should not rise above 2.5 mIU/ L during pregnancy. Hypothyroid patients undergoing treatment (receiving thyroid hormone replacement medication) who are planning a pregnancy should have their dose adjusted to optimize serum TSH values. Lower preconception TSH values reduce the risk of TSH elevation during the first trimester.


T4 and T3: 

These are actual thyroid hormones acting at cellular and subcellular levels (nucleus and mitochondria) regulating the basal metabolic rate. A low level of T4 or T3 can produce symptoms arising from low cellular energy production. One should ensure that the T3 and T4 readings are in the middle of the normal range as that normal range is set so broad.  The thyroid gland excretes 85% of T4 and only 15% of T3. T4, which persists in the blood for several days, acts as a reservoir, providing the body with a continual supply of the much shorter-lived T3. T3 is four times more potent than T4 and it controls metabolic rate at the cellular level by modulating the rate of energy consumption in virtually every tissue in the body. The body tissues convert the low potency thyroid hormone T4 into the high potency thyroid hormone T3. So, this tissue-level control of T4-to-T3 conversion is potentially a huge determinant of the basal metabolic rate. The prescribed thyroid replacement medications (T4) such as Synthroid (levothyroxine) assume that it will convert into T3 but patients need to be tested to check if that is occurring as it is not a given. The enzyme that converts T4 into T3 is selenium-dependent.


Reverse T3 (rT3):

T4 can be converted into reverse T3 (rT3), which has no thyroid hormone activity at all, and which undercuts the basal metabolic rate.  If no measurements are taken of T3 and rT3 levels, we have no idea of the relative contribution of activation and deactivation of the thyroid hormone.


Thyroid antibody testing is discussed in a separate article (Link)

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