Thyroid hormones

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Thyroid hormones

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 kidneys) to T3 that has stronger effects.

Thyroid hormones have an important function 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 insufficiency (for example deficient thyroid-gland activity or hypothyreosis) leads to the retardation of metabolism, growth and development. T4 (levothyroxine) is a prescription drug that is used for the treatment of hypothyreosis. The dosage is individual-based.

The effect of excessive thyroid hormones is accelerated metabolism (becoming thinner), anxiety, mental imbalance and sometimes heart arrhythmias. An excessive effect can appear, for example, if a healthy person uses a drug that contains thyroid hormones or if the drug dosages of a person suffering from hypothyreosis are increased too quickly.


The aim is to shape body composition using thyroid hormones to accelerate metabolism and burn body fat [1].

Adverse effects

In patients who suffer from hypothyreosis, thyroid hormone drugs have mainly positive effects as long as the dosage is not increased too quickly and is not oversized, in which cases symptoms that are similar to hyperthyreosis appear, such as heart palpitation, arrhythmias, diarrhea, emaciation, decrease in temperature tolerance, insomnia, agitation, and even psychoses [2, 3].

For people with latent heart diseases, adverse heart related effects (arrhythmias, heart infarct) can be perilous [4]. Before starting a thyroid gland treatment, the doctor should carefully review the patient’s health including the functioning of the heart. The risks related to the treatment can thus be determined.

Prolonged thyroxine overdosing increases the risk of osteoporosis. Perilous agranulocytosis (lowered white blood cell count) is found in 0.1 to 0.5 percent of patients [5].

Acute large thyroxine doses lead to thyrotoxicosis. The symptoms are high body temperature, failing heart functioning and coma [6]. Overdosing of thyroid gland hormones have led to deaths.

Timo Seppälä
Medical Director
The Finnish Antidoping Agency FINADA

[1] McKillop (1987): Drug abuse in body builders in the West of Scotland. Scottish Medical Journal 32(2): 39–41.

[2] Barczyński, Thor, Słowiaczek & Pitala (2000): The role of the autonomic nervous system on malfunction of gastric motor and myoelectric activity in patients with hyperthyroidism. Folia Medica Cracoviensia 41(3–4): 87–112.

[3] Hepp (1978): Psychiatric and neurologic disturbances in thyroid disorders. Wiener Klinische Wochenschrift Suppl. 94: 14–6.

[4] Nagele, Hörmann & Nagele (2009): First case study of periodic hypokalemic thyreotoxic paresis with partial respiratory insufficiency and concomitant sinus bradycardia. Wiener Medizinische Wochenschrift 159(5–6): 156–9.

[5] Bartalena, Bogazzi & Martino (1996): Adverse effects of thyroid hormone preparations and antithyroid drugs. Drug Safety 15(1): 53–63.

[6] de Luis, Abad, Aller, González-Sagrado & Dueñas (2004): Levothyroxine poisoning: clinical manifestations and therapeutic management. Anales de Medicina Interna 21(1): 39–41.

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