Classification of agents affecting thyroid function
All medicinal substances affecting the functional activity of the thyroid gland can be divided into two main groups that exert directly opposite effects on the patient's body:
- Thyroid hormone preparations — used as replacement therapy for hypofunction.
- Antithyroid agents (thionamides) — used to suppress excessive hormone synthesis.
Replacement therapy: hormone preparations
This group of drugs is intended to compensate for the deficiency of endogenous thyroid hormones. Synthetic analogues of natural compounds are used in clinical practice:
- Levothyroxine — a synthetic analogue of the T4 hormone.
- Liothyronine — a synthetic analogue of the T3 hormone.
- Combination preparations — contain several active components simultaneously to more closely mimic the natural secretion of the gland.
Antithyroid agents (Thionamides)
Antithyroid drugs are prescribed for conditions accompanied by thyroid hyperfunction. The main representative of this group is Thiamazole (also known by the brand name Mercazolil).
Mechanism of action: The action of thiamazole is based on inhibiting the activity of a specific enzyme, thyroid peroxidase. Blockade of this enzyme leads to disruption of the iodination of tyrosine residues within the thyroglobulin protein molecule. As a result of this cascade of reactions, the synthesis of the main thyroid hormones — T3 and T4 — is critically impaired and halted.
Side effects: goitrogenic effect
The most characteristic and clinically significant side effect of antithyroid drugs (particularly thiamazole) is the goitrogenic effect — a pathological enlargement of the thyroid gland leading to the formation of goiter.
Mechanism of the goitrogenic effect: The development of goiter is explained by the physiological principle of negative feedback. During the administration of antithyroid drugs, there is a sharp drop in the levels of circulating thyroid hormones (T3 and T4). The body responds to this deficiency with hypersecretion of TSH (thyroid-stimulating hormone) by the pituitary gland. Excess TSH exerts a powerful stimulating effect on thyroid tissue, provoking its compensatory growth and hyperplasia.
Briefly: answers to topical questions
What are the indications for prescribing levothyroxine, apart from replacement therapy for hypothyroidism?
- Suppressive hormone therapy (levothyroxine sodium suppressive hormone therapy) — indicated in intermediate-risk groups, biochemical recurrence, and indeterminate tumor status in differentiated thyroid cancer to improve survival rates.
To memorize the mechanism of antithyroid drugs: Thiamazole Inhibits Thyroid peroxidase — T3 and T4 are Terminated — TSH Triggers Tissue growth (goiter).
Class and exam questions
What drugs are used for the replacement therapy of thyroid diseases?
Replacement therapy utilizes levothyroxine (synthetic T4), liothyronine (synthetic T3), and their combinations.
What is the mechanism of action of thiamazole?
Thiamazole inhibits the enzyme thyroid peroxidase, which blocks the iodination of thyroglobulin tyrosine residues and disrupts T3 and T4 synthesis.
Why can goiter develop when taking antithyroid drugs?
A decrease in T3 and T4 levels causes pituitary TSH hypersecretion via a feedback mechanism. High TSH levels stimulate the proliferation of thyroid tissue (goitrogenic effect).
What else to study on this topic
- The role of thyroglobulin in hormone storage
- The relationship between ACTH and endocrine regulation
- Pharmacokinetics of levothyroxine and liothyronine
- Features of the use of combined T3 and T4 preparations
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