The thyroid gland in the neck makes hormones that set the body’s metabolic rate, which governs how energy is produced or stored and how waste products are removed. Thyroid hormone levels affect the behaviour of all cells and organs in the body, including the ovaries and testes. Thyroid disorders can be a crucial issue for couples who have 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:
- 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)
- The pituitary produces thyroid-stimulating hormone (TSH) (also called thyrotropin), which stimulates the thyroid to produce thyroid hormones
- The thyroid produces a variety of thyroid hormones (T1– T4)
When the thyroid hormones reach 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 vary according to the number of iodine molecules attached to them, which also affects how long they last in the body, and how biologically active they are, and T4 (thyroxine) is usually used as the marker for all of the thyroid hormones.
- 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 made in two ways; either directly by the thyroid gland, or it’s converted from T4 when an iodine molecule is removed. Some people make sufficient T4 but aren’t good at converting it to T3, which is the most powerful type. A “Reverse T3” hormone also exists, and this is crucial for some people, and the issues of conversion from T4 can be due to a lack of micronutrients in the diet.
All hormones have a “half-life”, which is how long they last before half of them 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
There are two types of thyroid disorders (either over-or under-active), and the imbalances can be either overt or subclinical:
- ‘Overt’ is the more extreme (and universally accepted form) where both TSH and T4 levels are abnormal
- ‘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 affects the chances of miscarriage and children’s health.
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)
The thyroid gland isn’t producing enough thyroid hormone relative to the stimulation it gets from TSH. The symptoms for women are:
- Abnormal menstrual cycles (usually with heavy periods)
- An increased chance of miscarriage and complications for the baby
- Puberty may be disrupted
Thyroid testing measures TSH and thyroid hormones, and it’s not unusual for T4 to be the only thyroid hormone tested; however, there are tests 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. Thyroid hormone production varies dramatically across the day, and levels are often twice as high at the night as in the afternoon and the most accurate time to take readings is in the morning. iv
ii ’Subclinical Hypothyroidism Is Mild Thyroid Failure and Should be Treated ‘Michael T. McDermott, E. Chester Ridgway The Journal of Clinical Endocrinology & Metabolism October 1, 2001 vol. 86 no. 10 4585-4590
iii “Thyroid pathology” Jack DeRuiter (2002). Endocrine Module (PYPP 5260). Auburn University School of Pharmacy. p. 30.
iv “Free Triiodothyronine Has a Distinct Circadian Rhythm That Is Delayed but Parallels Thyrotropin Levels” W. Russell Et al. The Journal of Clinical Endocrinology & Metabolism, Volume 93, Issue 6, 1 June 2008, Pages 2300–2306, https://doi.org/10.1210/jc.2007-2674
Photo by William Priess on Unsplash