Introduction:
Thyroid hormone is essential for the well-being of all body cells. Insufficient thyroid hormone secretion leads to hypothyroidism, often due to Hashimoto’s disease or other causes like surgery, radiation, or pituitary issues. Unlike most medications, thyroid hormone replacement aims to substitute a deficient hormone. The treatment’s objective is to mimic normal thyroid function closely. Levothyroxine, biologically akin to human thyroxine (T4), is the common choice for thyroid hormone replacement and is found in tablets, gel capsules or liquid forms. Safety revolves around avoiding over or under-dosing. Regular medical supervision ensures thyroid levels remain within the normal range.
What Is Thyroid Hormone Therapy?
Thyroid or replacement therapy involves utilizing synthetic thyroid hormones to elevate deficient natural thyroid hormone levels in the body. Administered as oral pills, this treatment frequently addresses an underactive thyroid, producing insufficient thyroid hormones. The primary choice for thyroid hormone replacement is synthetic thyroxine (T4) or Levothyroxine.
What Are the Indications for Thyroid Hormone Therapy?
Thyroid hormone is commonly recommended when the thyroid fails to produce sufficient hormone naturally and is indicated in the following cases:
-
To manage the enlargement of the thyroid gland (goiter).
-
To regulate the growth of nodules on the thyroid gland.
-
Following the removal of the thyroid due to benign or malignant conditions.
-
After treating hyperthyroidism with radioactive iodine ablation.
-
To hinder the continued expansion of thyroid tissue (suppression therapy- in thyroid cancer patients to prevent recurrence).
How Is Thyroid Hormone Dosage Decided?
Healthcare professionals perform blood tests to determine the appropriate dosage for hormone replacement therapy tailored to each individual. The initial dosage is carefully determined when initiating thyroid hormone treatment, considering age, weight, and health conditions.
Adjustments to the dose might be necessary over time to maintain normal thyroid levels, monitored through physical examinations, and the physician measures TSH (Thyroid Stimulating Hormone) secreted by the pituitary gland. The pituitary gland’s role is pivotal in thyroid function, producing TSH that prompts thyroid hormone release. Elevated TSH levels may suggest an underactive thyroid or the necessity to adjust the dosage and hormone therapy.
Regular laboratory tests will gauge thyroid hormone and TSH levels. Hypothyroidism can be a progressive condition so periodic dosage adjustments might be required over time. To ensure the effectiveness of thyroid hormone therapy, one must consider the following:
-
Schedule regular appointments with the doctor.
-
Take thyroid medication at least one hour before breakfast and any calcium or iron supplements or at bedtime, and ensure a gap of at least three hours after eating or taking such supplements.
-
Inform the doctor about thyroid hormone treatment before starting any other treatment. Some treatments for different diseases can influence the required thyroid hormone dosage.
-
In case of pregnancy, keep the doctor informed about it.
-
If any new symptoms arise, let the doctor know immediately.
Different brand names are available for thyroid hormone, all containing Levothyroxine (T4) but with varying inactive ingredients. Generally, consistency is advised in sticking with the same brand or manufacturer, even for generics. If a change is noticed, informing the physician immediately and reassessing thyroid levels is important.
What Are the Various Ways of Thyroid Hormone Replacement Therapy?
Thyroid Formulations: The market offers various branded and generic versions of L-T4. Several studies have assessed the bioequivalence. Dong et al. observed that four such preparations met FDA criteria, were largely interchangeable for most patients, and have faced several criticisms. For optimal dosing, it is important that patients consistently receive the same preparation. If changes are needed, close monitoring of serum TSH levels is vital, especially for those with thyroid cancer and pediatric patients.
Tirosint: In recent times, there has been an emergence of soft gelatin capsules known as Tirosint (L-T4 sodium capsules). These capsules contain dissolved L-T4 in a gelatin, glycerin, and water mixture. Insights from a retrospective study indicate that these capsules offer a more advantageous absorption than conventional L-T4 tablets. This formulation benefits individuals with altered gastritis, lactose intolerance, or those using histamine H2 receptor blockers and proton pump inhibitors.
Thyroid Extracts: Desiccated thyroid or thyroid extract refers to preparations derived from the thyroid glands of domesticated animals. The most prevalent form, Armour Thyroid, originates from pigs and combines T3 and T4 hormones. A key issue with these preparations revolves around the T3 content. The T4 to T3 ratio in desiccated thyroid is 4.2:1. Significantly lower than the human thyroid’s natural secretion ratio of 14:1. This elevated T3 proportion results in above-normal T3 levels. Additionally, since T3 has a shorter half-life, its levels fluctuate throughout the day, peaking shortly after dosing. This raises concerns about potential thyrotoxicosis if thyroid extract therapy is not adjusted based on serum TSH levels. These elevated T3 levels are especially worrisome for patients using thyroid extract therapy in suppressive therapy for thyroid cancer.
Liothyronine: L-T3 tablets are available under Cytomel in the United States. Using synthetic L-T3 therapy has the advantage of bypassing the T4-to-T3 conversion step. However, it is uncertain how effectively the body’s tissue can utilize this T3, leading to a lack of physiological rationale. Additionally, direct treatment with L-T3 does not allow for regulated, time-specific, and tissue-specific production of T3 through peripheral deiodination of T4. In the case of T3 monotherapy, multiple daily doses are needed to maintain serum T3 levels due to the shorter half-life of L-T3 compared to L-T4. Due to the wide fluctuations in serum levels resulting from T3 administration, its use is no longer recommended except for short-term use in preparation for radioiodine treatment.
What About Thyroid Hormone Therapy for Thyroid Cancer?
Following thyroidectomy for thyroid cancer, thyroid hormone therapy is essential to compensate for the missing thyroid hormones and to indirectly inhibit tumor growth through its impact on pituitary TSH secretion via harmful feedback mechanisms. Differentiated thyroid cancer features the TSH receptor on its cell surface, responding to TSH stimulation by upregulating thyroid-specific proteins and accelerating cell growth. Utilizing higher-than-normal doses of L-T4 to suppress TSH is a commonly employed approach to manage thyroid cancer patients, aiming to reduce the risk of recurrence.
The findings suggest that TSh suppression therapy only reduces recurrence rates and mortality in patients with high-risk differentiated thyroid cancer. Despite the correlation between low TSH levels and decreased thyroglobulin levels, pushing TSH levels below 0.5 mU’L does not result in a further reduction. This suggests that aggressive TSH suppression may offer minimal benefits in limiting tumor growth. The role of TSH suppression during long-term thyroid hormone therapy follow-up for differentiated thyroid cancer patients remains uncertain. Limited data exist on the relationship between varying serum TSH levels and the risk of recurrence and death from thyroid cancer at six to 12 months post-surgery and radioactive iodine ablation.
The current ATA (American Thyroid Association) guidelines persistently maintaining TSH levels below 0.1 mU/L is recommended for patients with ongoing disease unless specific contraindications exist.
Conclusion:
Thyroid hormone replacement therapy is crucial for addressing conditions like hypothyroidism, thyroid cancer, and thyroidectomy. Careful evaluation and medical supervision should guide the choice between different formulations and alternatives. While TSH suppression has proven effective in reducing recurrences and mortality rates, its benefits appear most significant for high-risk thyroid cancer patients. Maintaining a delicate balance in TSH levels is vital, with the guidelines recommending tailored individual approaches.
