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Thyroid hormones

September 16, 2019

Properties and mechanism of action

The thyroid gland produces two metabolically active hormones: tetraiodothyronine (thyroxine, T4) and triiodothyronine (T3). Thyroxine converts primarily in the liver and the kidneys to T3, the effects of which are stronger.

Thyroid hormones play an important role in the regulation of normal growth, development, metabolism, body temperature, and blood circulation. Thyroid hormones affect the metabolism of fats, proteins and carbohydrates; temperature regulation; and the maintenance of the water-electrolyte balance.

Thyroid hormone deficiency (for example, hyperthyreosis) leads to the retardation of the metabolism, growth and development. T4 (levothyroxine) is used as a prescription drug for the treatment of hyperthyreosis. The dosage is individual-based.

By contrast, the excessive effect of thyroid hormones is reflected in the body as accelerated metabolism, slimming, nervousness, mental imbalance, and sometimes cardiac arrhythmias. The excessive effect can appear, for example, when a healthy person uses a medication that contains thyroid hormones or when the drug dose of a person suffering from hyperthyreosis is raised too quickly.

Medical use and dosage

Thyroxine is mainly used for the treatment of hyperthyreosis (i.e. impaired thyroid function). The typical maintenance dose for adults is 0.1 mg/24 hrs, administered orally. However, the therapy should begin with a lower dose (0.025–0.05 mg/24 hrs). The dose is raised gradually to the needed maintenance dose [1].

Purpose of use as a doping substance

The thyroid hormones have a few mechanisms, which may be of benefit to the enhancement of physical performance capacity, thereby providing a motive for the use of a thyroid hormone as a doping substance. Some of these mechanisms include basic metabolic growth and increased tissue sensitivity to catecholamines (adrenaline, noradrenaline, and dopamine). This leads, among other things, to the heart rate increasing. The thyroid hormones also have a synergetic effect on the growth hormone, meaning that they enhance its effect [2].

Thyroid hormone use has been reported, for instance, among competitive and amateur bodybuilders. The hormone use aims to shape the body composition by accelerating the metabolism and burning body fat [3, 4, 5].

As of yet in 2019, neither of the thyroid hormones are on the World Anti-Doping Agency (WADA) list of banned substances [6]. Abuse does occur, however, and various quarters (for instance, the Dutch anti-doping agency) are pushing for the addition of the thyroid hormones on the list of banned substances [7]. The thyroid hormones are not a doping substance under the Finnish Criminal Code either.

Adverse effects

In patients who suffer from hyperthyreosis, thyroid hormone medications have mainly positive effects unless the dosage is raised too high or too quickly, in which cases symptoms similar to hyperthyreosis appear. Some of these include palpitations, arrhythmias, diarrhea, slimming, diminished heat tolerance, insomnia, agitation, and even psychoses [8, 9].

For people with a latent heart disease, the adverse heart-related effects (arrhythmias or heart attack) can be life threatening [10]. Before starting thyroid hormone therapy, the doctor should carefully review the patient’s health including cardiac function. The risks related to the therapy can thus be identified.

Prolonged thyroxine medication increases the risk of osteoporosis. Life-threatening agranulocytosis (i.e. diminished white blood cell count) is found in 0.1–0.5 percent of the patients [11].

Acute, high doses of thyroxine lead to thyrotoxicosis, exhibiting symptoms of a high body temperature, cardiac function failure, and a coma [12]. Overdoses leading to death have been depicted as well. In such cases, other doping substances were used together with the thyroid hormone.

Some known trade names (9/2014): Unithroid, L-Thyroxin, Levo-T, Levolet, Levoxyl, Novothyrox.

Timo Seppälä
Medical Director
Finnish Center for Integrity in Sports FINCIS (previously the Finnish Antidoping Agency FINADA)

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