Elevated level of thyroxine (T4) represents increased serum levels of the hormone thyroxine, one of the thyroid hormones, the other being triiodthyronine (T3). Thyroid hormones regulate virtually every function of the body (Pic. 1) by increasing cellular metabolic rate, regulating protein synthesis and influencing the body’s response to certain hormones, such as catecholamines (i.e. adrenaline and noradrenaline). The condition of elevated thyroid hormones, either T3, T4, or both, is known as hyperthyroidism. 

Elevated T4 level is diagnosed by drawing a blood sample and testing it for the amount of T4. Normal levels of T4 in adults are usually from 5 to 12 micrograms per decilitre. Values exceeding this reference range are considered an elevated thyroxine level. Sometimes, some other tests are needed, such as the level of T3 or TSH (thyroid-stimulating hormone), to diagnose a thyroid condition or to find its cause.

Hyperthyroidism, the condition characterised by elevated T4 levels, may be caused ether by diseases of the thyroid gland itself (so-called primary hyperthyroidism), or diseases of the pituitary gland (secondary hyperthyroidism) or hypothalamus (tertiary hyperthyroidism) causing disruption in regulation of the thyroid gland activity. The most common cause of primary hyperthyroidism is Graves disease, an autoimmune disorder in which auto-reactive antibodies (antibodies attacking the body’s own cells) bind to receptors on thyroid gland cells and overstimulate them into production of excessive amounts of thyroid hormones. Other causes include toxic thyroid nodule and thyroid adenoma, abnormally active lumps of thyroid gland tissue. Secondary hyperthyroidism may be caused by hormone-secreting tumours of the pituitary or hypothalamus.

Hyperthyroidism more commonly affects women (approximately 2% of adult population) than men (0,2%). Because of the thyroid hormones’ effect on metabolism and heat production, a patient suffering from hyperthyroidism will usually have increased body temperature and will not tolerate overheated environments. Furthermore, hyperthyroidism affects almost every organ system. Other common symptoms include increased heart rate, bulging out of the eyeballs (exophtalmos, Pic. 2), dry skin, brittle hair, loss of weight, tremor, nausea, vomiting, diarrhoea, increased psychomotoric tempo and agitation. 

Hyperthyroidism is usually treated with thyreostatic drugs which reduce the function of the thyroid gland. For patients that do not respond to the treatment, surgery removing a part of the thyroid gland (subtotal thyroidectomy) or the whole gland (total thyroidectomy) is recommended.

Hyperthyroidism is also associated with conditions negatively affecting fertility in both females and males, including:

Menstrual irregularities

Women affected with hyperthyroidism may present with abnormalities of their menstrual cycle, most commonly light menstruation (hypomenorrhea), irregular periods, missed cycles, or completely absent menstrual cycle (amenorrhea). Although women with hyperthyroidism are usually able to conceive, hyperthyroidism may eventually lead to anovulation (condition when no oocyte is released during a menstrual cycle) and infertility. However, treatment and normalization of thyroid hormone levels usually restores the patient’s fertility.

Adverse pregnancy outcomes

Pregnant women with hyperthyroidism have an increased risk of pregnancy complications, such as premature labour, early miscarriage, or low birth weight. They have also a higher risk of developing preeclampsia, a condition characterized by hypertension (high blood pressure), edemas and proteinuria (presence of protein in the urine).

Lower semen quality 

In males, hyperthyroidism may also significantly affect fertility by altering many semen characteristics. In hyperthyroid male patients, low levels of bioavailable testosterone (testosterone that is able to affect its target tissues) and conversely, higher amounts of estrogens have been found. These hormonal changes negatively affect sperm (Pic. 3) maturation and semen quality, possibly leading to oligozoospermia (low sperm count) and decreased sperm motility.


Overview of the Thyroid Gland ―sourced from Boundless licensed under CC BY-SA 4.0
Hyperthyroidism ―by Queensland Government licensed under CC BY 3.0
Thyroid system ―by Mikael Häggström licensed under CC0 1.0
Exophtalmos ―by Internet Archive Book Images licensed under CC0 1.0
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