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Thyroid Disorders

The thyroid gland in the neck makes several hormones that set the body’s metabolic rate, which governs energy production in cells, energy storage, and the removal of waste products. Thyroid hormone levels affect how all cells and organs in the body behave, including the ovaries and testes. An awareness of thyroid disorders is crucial for all couples who’re having difficulty conceiving

“because of its significant, frequent and often reversible or preventable effect on infertility” i

Thyroid hormones regulate the function of all cells and are particularly important for controlling:

  • Energy requirements and the metabolic rate
  • Protein production and regulation
  • Cholesterol production and regulation
  • Sensitivity to other hormones
  • The use of vitamins, carbohydrates, fats, electrolytes
  • The growth and function of multiple systems in the body
  • Aspects of heart functions

The thyroid loop

The thyroid is part of the hypothalamic-pituitary-thyroid (HPT) axis, and the glands in the axis work in a sequence to manage the metabolism with thyroid hormones:Mikael Häggström

  1. The hypothalamus releases thyrotropin-releasing hormone (TRH), which stimulates the pituitary to produce TSH. (High levels of TRH can also stimulate the pituitary to produce ‘prolactin’ and stop menstrual cycles: hyperprolactinaemia)
  2. The pituitary produces thyroid-stimulating hormone (TSH) (also called thyrotropin), which stimulates the thyroid to produce thyroid hormones
  3. The thyroid produces many types of thyroid hormones (T1– T4)

T4 (thyroxine) is usually used as the marker for all of the thyroid hormones. When the thyroid hormones are at the right level, they stop the hypothalamus from producing further TRH (and overheating the body) in a ‘negative feedback system’ similar to how the menstrual cycle works.

Thyroid hormones

Thyroid hormones vary according to how many iodine molecules are attached, which gives them their numbers. They also vary in how long they last in the body, the proportion made, and how biologically active they are:

  • T4 : 80% (Thyroxine) is the most common hormone
  • T3 : 15% (Triiodothyronine) is less common but about four times more “biologically active” than T4 (particularly for the liver and brain)
  • T2 : 3%
  • T1 : 1%

T3 is either made directly by the thyroid gland or converted from T4 by removing an iodine molecule. There are also “reverse T3” hormones which are crucial for some people. Some people can make T4 but aren’t good at converting it to T3, which can be due to a lack of micronutrients in the diet. All hormones have a “half-life”, which is how long it takes for half of them to degenerate:

  • T4 has a half-life of 7 days
  • T3 has a half-life of 1 day
  • TSH has a half-life of less than 1 hour ii

Thyroid disorders

There are two types of thyroid disorders (either over-or under-active), and the imbalances can be either overt or subclinical:

  1. ‘Overt’ is the more serious (and universally accepted form) where both TSH and T4 levels are abnormal
  2. ‘Subclinical’ is when T4 levels are normal, but TSH is abnormal
    • Overt Hyperthyroid has low TSH and raised T4 levels
    • Subclinical Hyperthyroid is milder: TSH is low, but T4 levels are normal
      • Overt Hypothyroid has raised TSH and low T3 and T4 levels
      • Subclinical Hypothyroid is milder: TSH is high, but T4 levels are normal iii

Although subclinical hypothyroidism has normal T4 levels, there are problems with TSH production, and clinical outcomes can be very similar to the overt form. This is particularly important for women trying (or pregnant) as low TSH can affect the viability of a pregnancy and the health of offspring.

  1. Hyperthyroid (overactive)

The thyroid is producing excessive thyroid hormones in response to the level of stimulation it gets from TSH. For women, the symptoms are:

  • Oligomenorrhea (no periods)
  • Anovulation (not ovulating)
  • Increased bleeding (rare)
  1. Hypothyroid (underactive)

The thyroid gland isn’t producing enough thyroid hormone relative to the stimulation it gets from TSH. The symptoms for women are:

  • Anovulation
  • Abnormal menstrual cycles (usually with heavy periods)
  • An increased chance of miscarriage and complications for the baby
  • Puberty may be disrupted

Thyroid testing

Thyroid testing measures TSH and thyroid hormones, and it’s not unusual for T4 to be the only thyroid hormone tested. Tests are available that include T3 and reverse T3.

It’s possible to test thyroid hormones with urine samples, and children or pregnant women are recommended to use urine iodine concentration (UIC) tests. Recent evidence suggests thyroid hormone production varies dramatically across the day, and levels are often twice as high at the night as in the afternoon. The most accurate time to take readings is in the morning. iv

For more information, see symptoms, references and risk factors and hypothyroidism and infertility.


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