The thyroid is an endocrine gland present in the front lower part of the neck, just below the “Adam’s apple” or larynx. The function of the thyroid is to make hormones T3 (triiodothyronine), T4 (thyroxine) and calcitonin.

Thyroid cells absorb iodine from the bloodstream and combine it with the amino acid tyrosine to make T3 and T4. These thyroid hormones are released into the bloodstream and are primarily responsible for controlling the metabolic rate.

Normal Physiology of the Thyroid Gland

physiology-of-the-thyroid-gland

• The hypothalamus (part of the brain) secretes TRH (Thyrotropin-releasing hormone)
• TRH acts on the pituitary gland to release TSH (Thyroid-stimulating hormone)
• TSH stimulates the Thyroid gland to secrete T3 and T4
• T3 and T4 have a negative feedback mechanism. If there is an excess secretion of T3 and T4, then these hormones influence the hypothalamus and the pituitary gland to slow down – results in the reduction of TSH levels.

Significant changes in the maternal thyroid function during pregnancy

1. Pregnancy is a state of relative iodine deficiency because of:
• Active transport of iodine to the fetoplacental unit
• Increase iodine excretion in the urine (increased glomerular filtration rate)
• Increase in iodine absorption from the bloodstream by the thyroid gland
The World Health Organization recommends an increase in iodine intake from the standard 100 to 150 ug/day to at least 200 ug/day during pregnancy.

2. Increase in T4-binding globulin (TBG):
• TBG is one of the proteins that transport thyroid hormones in the blood
• TBG has the highest affinity for T4 and T3
• During pregnancy, the increase in estrogen induces increased production of TBG in the liver

3. Increase in Total T4 and Total T3 levels:
• Increase in TBG lowers the free T4 and free T3 concentration, which results in an elevated TSH secretion
• Increased TSH levels decrease the production of total T4 and total T3

4. Change in TSH levels:
• The placenta in humans secretes a hormone called hCG (human chorionic gonadotropin)
• During the first trimester, the hCG drives a significant fraction of the thyroid-stimulating activity, and the TSH levels decrease
• During the second and third trimesters, the TSH levels increase due to the increase in TBG

Hypothyroidism and Pregnancy

Hypothyroidism is a condition where the thyroid gland produces less amount of thyroid hormones – T3 and T4.

During pregnancy, the foetus requires some amount of the mother’s hormones for its growth and development. Hence, there is an increased demand for thyroid hormones for the first 12- 14 weeks post conception – that is until the baby’s thyroid gland starts producing hormones.

In the first trimester, it is essential for the mother’s thyroid gland to cope with the stress of making enough hormones for herself and her baby. In some cases, the mother’s thyroid may not make sufficient hormones and may develop an underactive thyroid which results in an elevated level in TSH.
Depending on the Free T4 and Free T3 levels, hypothyroidism can be categorised as Subclinical Hypothyroidism and Clinical Hypothyroidism.

Clinical and Sub-clinical Hypothyroidism

 

Clinical Hypothyroidism

Subclinical Hypothyroidism

Serum TSH (mIU/L)

High (>10)

High (>3 and <10)

Free T4

Low

Normal

Free T3

Normal or Low

Normal

 

Common Causes of Hypothyroidism

Insufficient iodine in the diet – The thyroid needs iodine to produce thyroid hormone. Since the human body does not make iodine, one can consume iodine from food sources like iodised table salt, shellfish, saltwater fish, eggs and dairy products.

Hashimoto’s thyroiditis – An autoimmune disorder of the thyroid gland where the body produces antibodies that attack the thyroid gland

Secondary hypothyroidism – an underactive thyroid that results from a problem with the pituitary gland

Tertiary hypothyroidism – an underactive thyroid that results from a problem with the hypothalamus

Radioactive iodine treatment – Some patients with an overactive thyroid gland (hyperthyroidism) undergo a radioactive iodine treatment. However, radiation can destroy the thyroid gland cells, and this may lead to hypothyroidism.

Medication – Certain drugs which consist of Ferrous Sulphate, Sucralfate, Cholestyramine, Aluminium Hydroxide can inhibit the GIT absorption of the thyroid hormone. It is advisable to maintain a gap of 4 hours between the consumption of the drugs mentioned earlier and the drug for the thyroid.

Symptoms of Hypothyroidism

Metabolism slows down:

  • Lethargy/fatigue
  • Weight gain
  • Cognitive dysfunction
  • Cold intolerance
  • Constipation
  • Bradycardia
  • Delayed relaxation of tendon reflexes
  • Slow movement and Slow speech

Deposition of matrix substances:

  • Dry skin
  • Hoarseness
  • Oedema
  • Puffy face and Eyebrow loss
  • Peri-orbital oedema
  • Enlargement of the tongue

Others

  • Decreased hearing
  • Myalgia and paresthesia
  • Depression
  • Menorrhagia
  • Arthralgia
  • Pubertal delay
  • Galactorrhea

Screening and Treatment

What do the ACOG (American College of Obstetricians and Gynecologists) guidelines say?

  • TSH and FT4 should be measured to diagnose thyroid disease during pregnancy
  • Treat overt hypothyroidism during pregnancy with adequate thyroid hormone
    • Monitor TSH levels
    • Adjust the dosage of thyroid hormone replacement accordingly
  • Treat overt hyperthyroidism during pregnancy with adequate thioamide
    • Monitor FT4 levels
    • Adjust the dosage of thioamide accordingly
  • In the first trimester, TSH should be < 2.5 mIU/L and < 3.0 mIU/L after the first trimester
  • Subclinical hyperthyroidism (an abnormally suppressed TSH accompanied by a normal FT4 level) is present in approximately 1.5% of pregnant women. No adverse outcomes have been associated with this finding, and so, it is not recommended to check thyroid function tests routinely.
  • Universal screening for thyroid autoantibodies in pregnancy is not recommended since there is no sufficient data to support any benefit of screening and treatment in pregnancy of euthyroid women.

Conclusion

  • It is advisable to evaluate thyroid function as part of the overall pre-conception evaluation.
  • Target maternal TSH concentrations during pregnancy should be below 2.5 mIU/L in the first trimester and <3.0 mIU/L after the first trimester
  • Pregnant women with high levels of TSH concentrations should check the TPOAb status and seek appropriate treatment
  • Isolated hypothyroxinemia (low FT4 with normal serum TSH) should not be routinely treated during pregnancy
  • TSH should be monitored every trimester until delivery

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