Thyroid hormones (thyroxine (T4) and triiodothyronine (T3) stimulate the production of specific enzymes to be made; this includes a key enzyme in energy production called cytochrome oxidase that is needed for cellular energy production in the electron transport chain of the mitochondria. The mitochondria are directly related to the availability of energy (ATP) in every cell to produce energy for cell growth and normal metabolic rates. This is why when thyroid hormone production is reduced, the clinical features are fatigue, cold intolerance, weight gain and constipation to name a few as the bodies metabolic processes slow down.
T4 is produced in much larger quantities by the thyroid gland and is converted into T3 in tissues in the body, which is up to 4 times more potent than T4.
In the hair follicle, a reduction of thyroid hormones results in excess shedding of hair and a dry, brittle texture, but conversely an excess of thyroid hormones can also produce excess hair shedding, a reduction in hair shaft diameter, brittleness but also with greasiness from the sebaceous gland over-stimulation.
Hair follicles express thyrotropin-releasing hormone (TRH) and thyrotropin (TSH), and have receptors to thyroid hormones T3 and T4. Here are some of the cellular mechanisms that these hormones impact on the hair follicle:
- T4 stimulates keratinocyte proliferation in the hair matrix
- T3 and T4 inhibit apoptosis of the keratinocytes in the hair matrix
- T3 and T4 prolong the duration of anagen
- TGF-B2 is inhibited by thyroid hormones (TGF-B2 inhibits anagen, interestingly this is upregulated in androgenic alopecia and in retinoid-induced hair loss)
- T3 and T4 stimulate hair follicle pigmentation
- T3 and T4 reduces reactive oxygen species formation and increased amount of ROS scavengers.
- T3 and TSH increase follicular heat production
- Beek N et al, (2008), Thyroid Hormones Directly Alter Human Hair Follicle Functions: Anagen Prolongation and Stimulation of Both Hair Matrix Keratinocyte Proliferation and Hair Pigmentation, The Journal of Clinical Endocrinology & Metabolism, Volume 93, Issue 11, 1 Pages 4381–4388,available from https://academic.oup.com/jcem/article/93/11/4381/2627273
- Marshall. W et al. (2017) ‘Clinical Chemistry; Chapter 11 The Thyroid Gland’, Elsevier UK
- Vidali. S et al (2013) ‘Hypothalamic–Pituitary–Thyroid Axis Hormones Stimulate Mitochondrial Function and Biogenesis in Human Hair Follicles’ Journal of Investigative Dermatology, Volume 134, Issue 1, 33 – 42. Available from: https://www.jidonline.org/article/S0022-202X(15)36455-1/fulltext