By: Kathryn C. Calhoun, MD, ACRM
Background
The thyroid gland is located in the base of the neck and plays a key role in metabolism, development and reproduction.
The thyroid gland is under the control of the pituitary gland, which secretes Thyroid Stimulating Hormone (TSH) in response to feedback from circulating levels of thyroid hormone and Thyroid Releasing Hormone (TRH) from the hypothalamus.
Dietary iodine is transported in the blood as iodide, which is taken up by the thyroid gland in response to TSH. Iodide is then combined with tyrosine to form thyroxine (T4) and triiodothyronine (T3) which are the thyroid hormones. T4 and T3 are stored as part of the thyroglobulin molecule within the thyroid until their release is stimulated by TSH.
T3 is more biologically active than T4. In fact, T4 is primarily a prohormone for T3, meaning that the body converts T4 to T3, according to its needs. T4 is more tightly bound to transport proteins in the blood to leave T3 free to effect biologic activity.
That’s great … but why do we care?
Both hypothyroidism (low thyroid function) and hyperthyroidism (high thyroid function) can have significant effects on health and fertility.
HYPOTHYROIDISM: low thyroid function/low thyroid hormones
This may be primary (due to thyroid gland dysfunction) in which case the TSH will be high and the thyroid hormone will be low – in other words, the brain is working overtime to get the thyroid gland to do its job. It may also be secondary (due to central hypothalamic/pituitary dysfunction) in which case both the TSH and thyroid hormone will be low – i.e. thyroid hormone is low and the brain cannot respond to compensate.
Often, the reason for hypothyroidism cannot be determined and does not impact treatment. Sometimes it is due to an auto-immune attack on the thyroid (Hashimoto’s thyroiditis) where the body attacks its own gland with antibodies. Hypothyroidism becomes more common as we age; women should be screened every 5 years starting at age 35 and every 2 years starting at age 60. Of course, if a woman is symptomatic, she should be screened immediately.
Symptoms of hypothyroidism:
- Mental slowness (memory, speech, attention)
- Decreased energy
- Cold intolerance or low body temperature
- Constipation
- Low-pitched voice
- Edema or water retention
- Menstrual irregularities
- Delayed reflexes
- Slow heart rate
Screening for hypothyroidism:
Unless there is suspicion for secondary hypothyroidism or evidence of other pituitary/hormonal abnormalities, your doctor will usually start with a TSH level. If this is high, a Free T4 level is checked to see if thyroid hormone production is adequate.
Subclinical hypothyroidism is when the TSH is high but the thyroid hormone level is still normal – this could reflect an acute/limited or a chronic/ongoing need for increased pituitary stimulation of the thyroid gland to produce adequate thyroid hormone. There is an increased risk of miscarriage for women with both clinical and subclinical hypothyroidism, so thyroid hormone replacement is indicated for women with subclinical hypothyroidisms who are trying to conceive.
Thyroid hormone replacement is best accomplished with synthetic T4 (thyroxine). Formulas that contain both T4 and T3 provide more T3 than the body needs; it is better to replace the T4 and allow the body to convert what it needs to T3. Once on replacement, adequacy of dosing should be check by monitoring TSH levels after 8 weeks (it takes the pituitary many weeks to reflect to changes in blood thyroxine levels.) In women of reproductive age, we often aim for a TSH level ? 2.50 uU/ml, as this level has been suggested to optimize reproductive outcomes. Medication requirements increase in pregnancy and frequent monitoring to assure the optimum TSH range is essential.
Thyroxine is not helpful in women with normal thyroid function and too much thyroxine can lead to osteoporosis (it increases bone re-absorption.)
HYPERTHYROIDISM: high thyroid function/high thyroid hormones; “thyrotoxicosis”
Common causes for an overactive thyroid include Graves disease (autoimmune attack) or Plummer’s disease (postmenopausal women with a history of a goiter), but can also include thyroid cancer. In hyperthyroidism, the TSH will be low and the Free T4 and/or T3 will be high.
Symptoms of hyperthyroidism:
- Nervousness, emotional instability
- Insomnia
- Heat intolerance
- Weight loss
- Fast heart beat, skipped heart beat
(“palpitations”)
- Menstrual irregularities
- Sweating
- Diarrhea
- Tremor
- Goiter (enlargement of the neck over the thyroid
gland)
Screening for hyperthyroidism:
Your doctor will usually start with a TSH level. If this is low, a T4 or T3 level is checked to see if thyroid hormone production is high. Your doctor may also check for antibodies and/or recommend a thyroid scan. A neck/thyroid exam will check for nodules, which may need to be aspirated or biopsied to rule out thyroid cancer, which is more common in women than men.
Subclinical hyperthyroidism is when the TSH is low but the thyroid hormone level is still normal – this is more common in women and African Americans. Progression to full hyperthyroidism occurs more frequently with lower TSH levels. Often, subclinical hyperthyroidism warrants evaluation but not necessarily treatment.
Treatment of hyperthyroidism:
Anti-thyroid medications decrease T4/T3 production. Beta-blockers may be necessary for patients with irregular heartbeats. Some patients may need radioactive iodine exposure, ablation or surgical treatments.
Bottom line …
The thyroid gland is important to our overall health, particularly reproduction. Thyroid problems are more common in women, and they increase with age.
Women should begin thyroid screening at age 35. Earlier screening is appropriate for women who experience any of the above symptoms, especially women with irregular periods or a history of miscarriage.
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