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Pharmacology · Additional topics

Side Effects of Levothyroxine

*Levothyroxinum natrium*

Levothyroxine is a synthetic analogue of the thyroid hormone thyroxine (T4), used for replacement therapy in hypothyroidism. The main adverse reactions associated with its intake are related to overdose, which leads to drug-induced thyrotoxicosis and requires strict monitoring of thyroid-stimulating hormone (TSH) levels via negative feedback.

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Pharmacological groupThyroid hormone preparations
Mechanism of actionSynthetic analogue of endogenous T4 hormone
Main riskOverdose and drug-induced thyrotoxicosis
Laboratory markerTSH level (low in case of drug excess)
4 min readFor medical studentsUpdated 10.10.2026
Material prepared by WhiteCoat Pro editorial team based on platform study guidesPending instructor review

Mechanism of action and classification

Levothyroxine sodium (Levothyroxinum natrium) belongs to the group of thyroid hormone preparations. In clinical practice, it is the mainstay of replacement therapy for hypothyroidism, a condition characterized by insufficient thyroid function.

The drug acts at the target organ level, replenishing the deficiency of endogenous thyroid hormones. Normally, the regulation of hormone synthesis occurs at multiple levels of the thyroid axis: thyrotropin-releasing hormone acts at the hypothalamic level (its pharmacological analogue is protirelin), and thyroid-stimulating hormone acts at the pituitary level (the drug thyrotropin), which stimulates the thyroid gland. Administration of synthetic T4 mimics the natural effects of the hormones, ensuring normal follicular growth, as well as adequate systemic synthesis and secretion.

Side effects: clinical presentation of overdose

Specific side effects of levothyroxine are primarily caused by exceeding the therapeutic dose. Excess drug causes a state of drug-induced hyperthyroidism (thyrotoxicosis).

Based on the clinical picture that antithyroid drugs (thyreostatics) counteract in true hyperthyroidism, synthetic T4 overdose leads to the following pathological changes:

  • Increase in basal metabolic rate: excessive stimulation of all metabolic processes in the body.
  • Uncoupling of oxidative phosphorylation: this biochemical disturbance is accompanied by a sharp increase in heat production.
  • Enhanced catabolism: disruption of anabolic processes leads to pronounced loss of muscle and fat mass.
  • Cardiovascular disorders: excess thyroid hormones cause tachycardia (increased heart rate).

It is important to note that, unlike thyreostatics (e.g., thiamazole), levothyroxine does not have a goitrogenic effect. It does not cause a compensatory enlargement of the thyroid gland (goiter), because it does not stimulate, but rather suppresses, the secretion of TSH by the pituitary gland.

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Laboratory monitoring and therapy adjustment

Since the line between therapeutic effect and adverse reactions directly depends on the dose, monitoring of levothyroxine therapy is performed strictly based on blood TSH levels. The control mechanism is based on negative feedback.

  1. Insufficient dose: if the TSH level decreases only slightly or remains high during drug intake, it means that the T4 concentration in the blood is insufficient for the full engagement of negative feedback. In this case, the levothyroxine dose is increased until the euthyroid state is achieved (full normalization of TSH within reference ranges).
  2. Overdose: an abnormally low TSH level indicates that excess T4 has excessively suppressed the pituitary gland. This is a direct signal of the risk of side effects and the need to reduce the dose.

Alternative drugs and data limitations

In addition to levothyroxine monotherapy, other agents may be used for replacement therapy, the side effects of which also depend on dosing accuracy:

  • Liothyronine (Liothyroninum) is a synthetic analogue of the T3 hormone.
  • Combination drugs — for example, Thyreotom, which contains a fixed combination: 40 mcg of levothyroxine and 10 mcg of liothyronine.
  • Animal-derived preparations — Thyreoidin. This drug is obtained from dried and defatted thyroid glands of slaughtered livestock. It contains levorotatory isomers of T3 and T4. Although its effect on metabolism is 3–5 times weaker than that of liothyronine, its indications, contraindications, and side effects are completely identical to those of synthetic T3.

It should be noted that accurately predicting side effects in clinical practice requires additional data. As part of a basic overview of the topic, specific dosages of levothyroxine in mg/day, detailed pharmacokinetic parameters (such as half-life and bioavailability), and specific drug interactions of thyroid hormones with anticoagulants and other drug classes are not disclosed.

How to memorize

The TSH and T4 "seesaw" rule: when the levothyroxine (T4) dose goes up and causes an overdose, pituitary TSH plummets (low TSH). If T4 is lacking (high TSH) — the seesaw requires adding weight, i.e., increasing the drug dose.

Class and exam questions

How to determine via laboratory tests that the levothyroxine dose is too high and causing side effects?

The main sign of overdose is an abnormally low TSH level. Excess synthetic T4 excessively suppresses pituitary TSH secretion via negative feedback.

What clinical symptoms indicate levothyroxine overdose?

Overdose manifests with symptoms of thyrotoxicosis: loss of muscle and fat mass, tachycardia, increased basal metabolic rate, and excessive heat production due to uncoupling of oxidative phosphorylation.

Does levothyroxine cause thyroid enlargement (goiter) as a side effect?

No, a goitrogenic effect is characteristic of antithyroid agents (e.g., thiamazole). Levothyroxine, conversely, lowers TSH levels, eliminating the stimulus for thyroid tissue growth.

What to do if TSH levels decrease only slightly during drug intake?

This means an insufficient T4 concentration for the full engagement of negative feedback. The levothyroxine dose must be increased until the euthyroid state is achieved.

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What else to study on this topic

  • Mechanism of action of antithyroid drugs (thiamazole) and causes of their goitrogenic effect.
  • Physiological iodine storage and the use of radioactive iodine in therapy.
  • Levels of regulation of the thyroid axis: hypothalamus (protirelin), pituitary (thyrotropin), and thyroid gland.
  • Comparative characteristics of synthetic T3 (liothyronine) and T4 (levothyroxine) preparations.
  • Symptomatic supportive therapy for hyperthyroidism with beta-blockers (propranolol).

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