Pre- and Post-Natal Growth: Effects of Thyroid and Steroid Hormones

Introduction to Pre- and Post-Natal Growth Factors

• Conceptual Overview: Maternal factors such as diet, stress, and sleep patterns have the capacity to alter pregnancy hormones in both the mother and the fetus.

• Core Focus: The primary focus of this study section is the impact of specific hormones—thyroid, adrenal, and reproductive (estrogens and androgens)—on fetal growth and development.

Thyroid and Steroid Hormones: Chemical and Receptor Characteristics

• Thyroxine ($T_4$):     * Composition: A small, tyrosine-based hormone.     * Transport: Because it is small, it requires carrier proteins for transport in the bloodstream.     * Classification: Explicitly noted as NOT being a steroid.

• Steroid Hormones:     * General Properties: Small, lipophilic (lipid-soluble) hormones.     * Mechanism of Action: They freely dissociate from their carrier proteins, pass through the cell membrane, and bind to cytoplasmic or nuclear receptors. Once bound, the complex translocates to the nucleus to activate transcription factors.     * Specificity: Most receptors are highly specific, although some precursors and related hormones can bind weakly to activate pathways.     * Examples: Sex steroids and glucocorticoids.

The Function and Regulation of Thyroid Hormones ($T_3$ and $T_4$)

• Essentiality: While the absence of thyroid hormone is not necessarily incompatible with life, hypo-function (hypothyroidism) during fetal and postnatal stages leads to significant developmental issues.

• Primary Roles:     * Maintenance of cell metabolism.     * Essential for long bone growth.     * Regulates temperature tolerance; animals with hypothyroidism exhibit both cold and heat intolerance.

• Hormonal Feedback Loop:     1. Hypothalamus: Produces Thyrotropin-Releasing Hormone ($TRH$).     2. Pituitary: $TRH$ stimulates the release of Thyroid-Stimulating Hormone ($TSH$).     3. Thyroid Gland: $TSH$ induces growth of the thyroid gland, promotes iodine uptake, and stimulates the production of Triiodothyronine ($T_3$) and Thyroxine ($T_4$).

• Activity Status:     * Thyroid hormones are typically bound to proteins in circulation.     * The "free" (unbound) form is the biologically active form.

• Human Specifics: In humans, which possess a hemochorial placenta, some $T_3/T_4$ may cross the placental barrier.

• Hypothyroidism Consequences: Slowed growth in children, specifically reducing long bone growth, which can lead to dwarfism (Reference: Hossner 2005, Hormonal Regulation of Farm Animal Growth).

Impact of Fetal Thyroid Hormones on Growth and Development

• Fetal Nutrition Link: Fetal levels of $T_3/T_4$ are affected by gestational age and maternal nutrition. Low levels are correlated with intrauterine growth restriction ($IUGR$) and undernutrition.

• Placental Transport: For many species, $T_3$ and $T_4$ cannot cross the placenta. Consequently, the fetal thyroid gland must develop early to produce its own hormones.     * The Hypothalamic-Pituitary-Thyroid ($HPX$) loop develops early, typically in the second trimester ($2^{nd}$ TRI).     * Hormones are detectable in the blood and amniotic fluid fairly early in gestation.     * All species demonstrate placental transport of Sodium ($Na^+$) / Iodine.

• Physiological Effects (Anabolic):     * Action is mediated through nuclear and cytoplasmic receptors.     * Thyroid hormones exert positive, anabolic effects on tissue growth.     * Fetal Sheep Studies (Thyroid Removal): Results of thyroidectomy include reduced birthweight, reduced organ weights, smaller skeleton size, and decreased protein content in the heart, muscle, and lungs. These effects can be remedied through $T_4$ infusion.

• Mechanisms for Growth:     * Increases Oxygen ($O_2$) consumption.     * Increases cardiac output.     * Requires the presence of Insulin-like Growth Factor ($IGF$) and insulin to achieve growth.

Steroid Hormones in Gestation: Glucocorticoids and Sex Steroids

• Chemical Relationship: These steroids are related by their shared precursor: Cholesterol ($C\text{-}27$).

• Circulation and Binding:     * Approximately $80\%$ of these steroids are bound to carriers like Corticosteroid-Binding Globulin ($CBG$) or Albumin (specifically for Progesterone/$P_4$).     * Only the free steroids are available to bind to cytoplasmic or nuclear receptors, such as the Progesterone Receptor ($PR$) or Glucocorticoid Receptor ($GR$).

• Progesterone ($P_4$):     * High levels are required to shift the maternal immune system toward tolerance of fetal tissues.     * Receptors are abundant in the uterus and placenta.

• Cortisol:     * Expressed in all cells and tissues, including immune cells, via the $GR$.     * In its free form, cortisol can cross the placenta.     * Stress can alter steroid levels because Progesterone and Cortisol share common biosynthetic pathways.

The Adrenal Gland: Structure and Steroidogenesis

• Adrenal Cortex:     * Deals with chronic stress/pregnancy demands to supply energy.     * Synthesizes Glucocorticoids ($C\text{-}21$ steroids) like cortisol or corticosterone (governed by species).     * Regulation: Controlled via a feedback loop involving the Hypothalamus ($CRH$) and Pituitary ($ACTH$).

• Adrenal Medulla:     * Deals with short-term, emergency energy needs (the "Fight or Flight" response).     * Produces epinephrine and norepinephrine.

• Metabolic Effects of Cortisol:     * Liver: Stimulates glycogenolysis and gluconeogenesis to increase blood glucose.     * Muscle: Promotes glycogenolysis and protein catabolism.     * Fat: Induces lipolysis (increases short-term energy but decreases fat stores in the long term) to mobilize alternative fuel and conserve glucose.

Sex Steroids and Their Role in Pre- and Post-Natal Growth

• Prenatal Effects:     * Determine sex-specific development of the reproductive system, typically around the $2^{nd}$ trimester.     * Testosterone (from testes) and Estrogen (from ovaries) act on fetal tissues to differentiate the system.

• Postnatal Effects:     * Produced at low levels prior to puberty to stimulate some tissue growth.     * Synergize with the $GH\text{-}IGF$ axis.     * High levels at puberty induce secondary sexual characteristics.

• Adrenal Androgens ($C\text{-}19$):     * Includes Androstenedione and $DHEA$.     * Possess less than $20\%$ of the activity of testosterone from the testes.     * Can induce anabolic growth when converted to testosterone or estrogen ($C\text{-}18$) in peripheral tissues (bone, fat, skin, muscle).

• Anabolic Androgenic Steroids ($AAS$):     * Androgens have a direct anabolic effect on muscle and bone growth through the Androgen Receptor ($AR$).     * Estrogens have indirect effects and do not act through the $AR$.

Sex Steroids and Bone Dynamics

• Endochondral Ossification: Long bone growth involves cartilage formation, vascularization, and eventual replacement by bone.     * Chondrocytes: Cartilage cells.     * Osteoblasts: Responsible for bone formation.     * Osteoclasts: Responsible for bone resorption.

• Receptor Presence: These bone cells possess both Androgen and Estrogen receptors.

• Epiphyseal Plate Closure: While adrenal androgens can stimulate growth prepubertally, high levels of estrogen at puberty cause the epiphyseal plates to fuse/close. Estrogen effects are cumulative and deplete the proliferative zone.

• Gender Differences: Growth and puberty generally occur earlier in females because estrogen levels increase earlier through the conversion of adrenal androgens in the ovary.

• Pathological Conditions:     * Precocious Puberty: Early exposure to estrogen causes bone growth to terminate prematurely.     * Estrogen Absence/Failure: In both males and females, a lack of estrogen (or receptor response) leads to normal growth until puberty, followed by excessive, prolonged long bone growth.

Performance and Growth Enhancers

• Testosterone and Derivatives:     * The testes produce $95\%$ of male testosterone.     * $5\text{-}\text{alpha }(\text{5a}) \text{ reductase}$: Converts Testosterone ($T$) into Dihydrotestosterone ($DHT$), which is $4X$ more potent.     * $DHT$ is non-aromatizable (cannot convert to estrogen) and is present in the prostate, skin, scalp, liver, and $CNS$.

• Anabolic Steroid Use in Humans:     * Used to build bone and muscle (requiring protein and exercise).     * Risks in Teens/Athletes: Early bone closure, behavioral changes, and undesirable side effects (females may experience androgenic effects; males may experience estrogenic effects due to conversion at high doses).     * Common Forms: Injections, pills, or creams (prescription required).

• Anabolic Steroids in Livestock:     * Used since the 1950s in cattle and sheep to improve growth efficiency and speed.     * Administered via ear implants or feed.     * Levels used are low; often no withdrawal time is listed, though export limits vary globally.