Thyroid issues: when hormones influence hair loss.

Each hair has its own life cycle composed of three phases :

• The anagen phase (2 - 6 years): This is the hair growth phase during which there is proliferation of matrix cells that form the inner sheath of the root, the cortex, and the medulla of the hair shaft. The synthesis and pigmentation of the hair shaft only occur during this phase. Its duration determines the length of the hair, generally 1 cm per month.

• The catagen phase (2 - 3 weeks): The first sign of the catagen phase is the cessation of melanin production, the pigment responsible for the colour of our hair, in the hair bulb. The hair stops growing but remains attached to its hair follicle.

• The telogen phase (2 - 3 months): After the catagen phase, the follicles rest in a dormant phase, the telogen phase. The hair shaft eventually detaches from its follicle, which is already starting to produce a new hair beneath the skin.

Thus, within a head of hair, the hairs are not necessarily in the same phase as they each have their own independent life cycle. Some may be in a growth phase, while others are on the verge of falling out. Their life cycles are dictated by hormonal or nutritional variations.

Clinically, it has been shown that T3 and T4 hormones increase the anagen phase of the hair cycle and have reduced the number of hairs in the catagen phase. They would prolong the anagen phase by down-regulating TGF-2, the main endogenous growth factor inhibiting anagen and also a recognised catagen inducer during the hair's life cycle. In the case of hypothyroidism, the thyroid does not produce enough T3 and T4 hormones. As a result, more hairs are found in the telogen phase, leading to visible hair loss. Depending on the severity of the hypothyroidism, hair loss can occur all over the body, with noticeable loss in the eyebrow area.

Generally, hair loss associated with thyroid disease can become more noticeable several months after the onset of the illness, due to the duration of the hair growth cycle.